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.