The 91 RS i just bought with the 305 seems to run just fine so far. I've only added ~150 so far but still it starts right up and runs smooth. There is no blue smoke out the back on start up and it doesn't appear to be burning any oil so I was wondering what type of oil to run when I change it. Should I run your standard 5w30 in this motor with 139k or is a heavier weight oil more advisable? 10w30, 10w40?

 

10W30 should be just fine.

 

i put in 10w40 in my 170k tpi 305...but it seems to love it

 

No smoke, no oil consumption here.....

I'm running 20W50 Castrol at 180k miles, but here in the Texas heat, the thicker oil isn't a problem, like it would be up north in the winter.

 

NW94Z, with the miles you have, I would run 10-40 summer and 10-30 winter. I`ve had good luck using 20-50 on high mileage motors when the temps are 100+ during the summer. In over 40 years of driving, I`ve never had any engine problems related to lubrication on sbc`s.

 

Thanks again for the input guys. Lot of different opinions. Sounds like 10-30 should be safe.



jimdutro,

The temps up here rarely average above 75 even in the summer. We'll have an occasional week or two with temps approaching 90 but it's not common. Would you still suggest 10-40 in the summer?

 

10-40 in summer and 10-30 in winter...consider switching to a synthetic blend or even straight synthetic...now many will come on and say you will develop leaks and the such but thatis not true. I switched to a blend at 85,000 and full synthetic at 132,000 and have no leaks or drop in performance....now at 143, 000 and counting

 

I'm not sure everyone here understands how oil weights work....

5W30 is what we typically use in the winter. That means that it's 5 weight when the engine is cold (cold meaning not running for a few hours), so it's thinner. So when the ambient temperature is COLD (winter) then it's thinner, and will pump better. It's 30 weight when the engine comes up to operating temperature.

10W30 is what we typically run in the summer. It will be 10 weight when the engine is cold (cold being ambient, that's still "cold" when talking about engines), and still 30 weight when it comes up to operating temperature. Down where you are, you don't really have "seasons" you have 4 summers back to back (gee, do I sound bitter about that at all?), so you don't even need to change oil weights.

The ambient temperature doesn't really matter once the engine is warm. The heat of combustion makes a "hot summer day" seem pretty insignificant. The radiator should be keeping the oil below a set temperature anyway. So I wouldn't run a super thick oil in a hot climate. It works fine for large clearance bearings, but otherwise it draws more HP.

EDIT : Re read where you are from. Washington eh? You have a climate like where i'm from, ie the Okanagan valley in BC there. The 5W30 / 10W30 is the "norm".

 

use full synth and just use 5w30 all the time, for your avg type driving.


edit: btw, i dont want people to think the oil thickens as it gets warm. It thins from the first number to the weight that the second number weighted oil would be at 100C. So it starts at 5 weight and ends at the weight a 30 weight oil would be at 100C. just read up here. http://auto.howstuffworks.com/question164.htm

 

I love this statement: "Multi-weight oils (such as 10W-30) are a new invention made possible by adding polymers to oil." {Emphasis added}

The owner's manual for the '57 recommends 10W-30 engine oil. . .

 

The higher the number the more viscous (more "thick") the oil is.
Oil does tend to thin out normally as it gets warmer, but multi-weight oils have additives that try to do the opposite. If you had straight 30wt oil, it would be like molasses, then thin out to "normal" at operating temp.
So instead you use a multiweight, and as it's thinning out normally, the additives are forcing it to stay more viscous, making it like 30wt when it's warm (even though it's only 10wt when cold).

-Interesting side note - industrial engines like to use straight weight oil, since it lasts longer, as the viscosity modifiers aren't there to break down.

 

NW94Z, given the temps where you live, 10-30 would be a good choice.

 

maybe they meant new in relation to the discovery of oil ?

in any case, what is _new_ is the use of group 4 synthetics, which dont need viscosity additives to maintain their "multi-weight" properties.

as for the location he's in. He's in washington, it's never really hot there. If we're gonna go suggesting oil based on ambient temp (for starting protection purposes), then he should use a 5w30. But either way, nothing bad will happen to his motor, the only downside to using 10w30 over 5w30 is some efficiency until it gets up to operating temp.

Much more important is what oil to use, unless he always changes at 3000 miles or sooner depending on driving conditions. If you can afford it, mobil 1 full synth or a group 4 full synth from amsoil can't be beat. No viscosity additives to break down and gives you plenty of lee way in oil change intervals.

 

I rarely go over 3000 miles between changes on any of my cars so as long as going synthetic will not cause any problems with springing a leak (can't see it happening) I'll start running Mobil 1 10w30.

Any opinions on 10w40. Does it exist and would a high mileage motor benefit from a thicker oil when warm since the tolerances on bearings and such are larger than they were when new?

 

you can't really tell without doing an oil analysis and seeing what is getting left in your motor oil when you do your changes. Worst case scenario is parasitic power loss by using heavier oils like 10w40, some lean towards that because it's safer than being far too thin. Everyone is just guessing unless they get the 20 dollar analysis done a few times and compare their mileage.

The temp outside only matters for the first rating, the second is rated at 100C, which is around the temp your coolant system should keep your car regardless of outside temps. Heavier weighted oils are usually needed for when the engine experiences much hotter operation (not from the outside, but the engine itself).

If you're just driving around and the engine isn't crazy modified, 40 may be overkill.

 

The oil takes a significant amount of heat out of the engine. Hot ambient temps may call for a thicker rated oil, but probably not in the original poster's case.

 

where you at, i don't see a need for anything thicker than 10W30 in the summer.
in the winter something thinner would be a good ideal. you want the oil to get to everything as fast as possible on cold start up.

it doesn't generally get very cold here in the winter, but into the 20s for a few days isn't uncommon.
on the other hand the summers get quite warm. 100+ starting in around mid June & lasting into September.
i have over 200,000 miles on my lower end & i run 10W30 year round.

 

i know this is an old thread, but at least i did a search before making a new thread.
so i have a question, i live in texas and it gets real hot here. well yesterday i was driving in 100 degree heat, and noticed when i stopped my oil pressure read low and my car acted funny like it was missing (i will be changing out my timing chain soon) but why would want to use a thicker oil in the summer? wouldnt it cause the motor to run harder therefore more heat then you already have?

 

DiabolicalRS, I`ve run well over 2 million miles on small block Chevys the past 40+ years without any engine failures due to lubrication. I use 10/30 the first 150k miles and usually switch to 10/40 or 20/50 at 150k+ depending on the temps where I live. My `88 Iroc has about 40k miles on a 383, the oil pressure is about 60psi at speed and drops to 20psi at idle. It`s normal for oil pressure to drop at idle, especially when it`s hot out. In your case I would try 10/40 or 20/40 and see how that works for you.

 

I have a 91 z28 with an lb9 305 with 195k on it . Engine is quite strong , not burning oil or smoking . Was thinking of using 10w40 in it , however the owners manual says do not use 10w40 as engine damage may occur . Why would it cause engine damage ? Any opinions/advice on this ?

 

I don't think many people know how thickness relates to oil and how the scale works.

For example, a OW-30, 5W-30, and 10W-30 will all have the same thickness of oil at full operating temperature (they're all 30 weight). But a 5W-20, 5W-30, and 5W-40 will NOT have the same thickness during a cold start. In these scenarios, the 5W-40 will be thicker than the 5W-30, which will be thicker than a 5W-20..........This means that a 5W-40 will generally be thicker than a 10W-30 during a cold start.

The hardest time for an engine to lubricate well is during a cold start. It's not so much because of the temperature. But that the colder temperatures mean the oil is waaaaaay too thick. Even a 0W-20 or for that matter a straight 0 weight oil is too thick for a cold start.

You generally want something with a thickness of 10-11 at full operating temperature. I'm not referring to the "10"W-30 numbers. Those are just a rating scale. I'm talking about the actual thickness of the oil. Take a look at the chart below to see what I mean.

(Concentrate on the Mobil 1 numbers in the top left for comparison purposes)



See how 0W-30, 5W-30, and 10W-30 all have the same thickness at full operating temperature (100°C)? They're all a thickness of "10."............Yet when you look at the cold start temperature (40°C), you'll see that they have much different thickness numbers depending on 0W-20, 0W-30, or 0W-40.

Now if we had a way to have oil be a thickness of 10 during a cold start (which we don't), then we could start up our cars and go full throttle first thing in the morning and have no more wear than we do when they're up to full operating temperature. It's a trade off. A 0W-20 oil would be the best for a cold start. But it's too thin at operating temp. A 0W-30 is better for operating temp, but it's thicker during a cold start than 0W-20.

As you can see, it's not really about thin vs thick. It's more like thick vs way too freaking thick.

Unless you live in Phoenix during the summer or some place like Saudi Arabia, you're not going to see a cold start at 40°C (104°F). In reality, it's going to be colder than that. Which means the oil will be even thicker still.

Mobil 1 oil at 10W-30 has cold/hot thicknesses of 62/10. Their 5W-30 is a bit better with 56/10. Their 0W-30 doesn't appear to have any benefit in terms of being thinner at cold start compared to 5W-30. Though it does have better ultra low temperature cranking and pouring capabilities. (Think Alaska, northern Canada, or even parts of the US like Minnesota)

Oil pressure always starts off high during a cold start. The oil is too darn thick! So it has to pump it harder. Just like a person who has clogged arteries will have higher blood pressure. Once the car gets up to temp, the oil thins out a bit. The oil pressure drops at this point to a more reasonable pressure. It's easier to pump the oil through the engine at this point.

10W-30 was fine for the 1960's and 70's. 5W-30 has been the preferred oil since the 1980's. In today's world, 5W-30 is still pretty good. Although in general, car manufacturers have been switching to lower numbered (thinner) oils. This is for fuel economy purposes as well as efficiency.

I could see running 10W-40 or 15W-50 in a race car that sees a lot of high RPM racing. Scenarios of extremely high temperature oil. In situations like this, the oil could become too thin, hence the need to thicken it up a bit with the higher __W-__ numbers. But for a street car? We just don't have those needs. Heck, look at the thickness numbers for Mobil 1 15W-50. They're 125/17. Molasses!

Now in terms of high mileage cars, some people say the tolerances open up. How much this is true or matters? I can't say. People who are losing oil through seals will often switch to a higher grade (from 10W-30 to 10W-40 for example) as a way to try to keep the oil from seeping through the seals. In other words, they're trying to make it thick enough that it won't leak.

"High Mileage" oils will promote themselves as having some sort of conditioner that is good for seals. But they'll also be slightly thicker than the regular versions. Comparing Mobil 1 synthetic vs Mobil 1 "high mileage" synthetic (both of them being 5W-30), the first has thicknesses of 56/10, while the high mileage one has thicknesses of 69/11.

If you're wondering, their high mileage 10W-30 synthetic oil has thicknesses of 78/12 (their regular 10W-30 synthetic being 62/10)


For Nanaimo's climate, stick with 5W-30 year round.

 

Thanks for the very useful information on oil weights . I have some oil coming through an intake manifold bolt which is why i was considering 10w40 . I will try and remove the bolt and apply some thread sealer to try and stop the leak. Hopefully can remove the bolt without removing other components on the intake.

 

Oldguy, had the same issue on several small blocks over the years. I removed the bolt where the leak was, clean the bolt and bolt hole as well as possible, coat threads with sealer and reinstall. Never had another problem with leaks. I use Loctite rtv 5920 copper or Permatex copper. 5/30 oil is a good choice for winter and 10/30 for summer. Are you using conventional oil or synthetic? I`ve read on this site that switching from conventional to synthetic oil on high milage engines can cause oil leaks. My Iroc with 40k miles on a 383 had always run conventional oil, no leaks, I switched to synthetic and after a year had to replace the rear main seal, major leak.

 

Jimdutro, thanks for your feedback . Just have one leaky bolt so far and it just started last summer. I have been using 10w30 Castrol conventional oil. The car is always stored for the winter months. Will remove the bolt and coat the threads with sealer as you suggested.

 

10-40 or 30

 

10-30

 

I have an original owners manual for my 1986 Trans Am here and on page 5-5 under choosing the right quality oil, it states, (An SF/CC quality, SAE 5W-30, Energy -Conserving oil is the preferred engine oil for your vehicle.)

 

NO !! the higher the mileage ,the thicker the oil . i run castrol GTX 20-50 in anything with over 100,000 miles. AND i advise nothing but WIX oil filters

 

10W-30, Quaker State Defy is my go to for high mileage oil.

 

Mine has low pressure with 10w30. I went to 10w40 and it helped a good bit to get pressure up.