well i bought a lt1 a month ago trying to finish the swap but would like to know something... the owner of the motor before me used conventional oil is it ok if i start using synthetic in the next oil change? i have heard that if u change it could blow your motor? also that synthetic expands tthe seals and gaskets and creates oil leaks? are these true? is it ok to start using syn? thanks

not really blow it, and NO IT DOESNT CREATE LEAKS lol sorry but some people feel there is some magic link between synhetic and CREATING leaks

Everyone knows that conventional oil build sludge from impurities in the mixture, it happens. That sludge tends to cake the seals and in most cases seals them better, when you switch to the new "clean" syntetic oil, hell yeah it cleans but too well, that sludge that kept your oil pan (or whatever) from leaking will be wiped away and the synthetic oil is so fine it will seep out

No harm in using it but be prepared for a leak here or there

synthetic oil is a bit overrated. the price of synthetic is pretty expensive when u compare it to conventional. although people get brainwashed into thinking synthetic oil is the way to go, there are some positives of synthetic oil, like it doesnt break down like conventional oil does, but then again if you take care of your car and check the oil from time to time this shouldnt be an issue. also its supposed to have better lubricating and cleaning properties this is good and bad at the same time since if youve neglected your oil in the past, sludgy oil may be whats keeping your rear main seal from leaking, and when you introduce the new synthetic oil, itll clean all the sludge and then youll have a leak.
be ready to burn alittle old too, because synthetic oil tends to be thinner, itll want to slip through places that have a significant amount of clearance. like in your cylinders. youll notice that your first couple of oil changes with synthetic, youll have to add alittle more after acouple of weeks or so. after your first couple of oil changes youll see you no longer burn oil. that is, if you keep using synthetic.

i recommend that if your just going synthetic because it happens to be cheaper this week, or theres a sale on it. just skip it. because you dont want to be switching between, non synthetic and synthetic all the time for the reasons just mentioned. what i use is a synthetic conventional blend, its cheaper than full synth. and happens that it isnt too expensive.

Personally I switched to synthetic oil in all of my cars. Switching from conventional oil to synthetic oil at a full oil change (with filter of course) will not be any danger to your engine.

The 350 engine in my 1979 Camaro is where I did my testing since it builds the most heat, and has aftermarket oil preasure guages that are far more accurate than stock. With standard oil i would notice a significant drop in oil preasure after about 1500 miles, almost 10 full psi. Also the older the oil got towards a change, the quicker it would lose preasure under normal to severe driving conditions. I found that after switching to synthetic oil my preasure is a lot more consistant through the entire 3000 mile life, and it looks a lot more healthy at the oil changes.

As far as how clean the oil is, it always depends on the brand. Castrol is known for having the highest consentration of detergents so it is traditionally the oil that will keep your internals the cleanest. There are others that are drastically not so clean. If you use one of those "dirty" brands and switch to a high detergent brand you can develop leaks as described by Gurk, weather you go synthetic or not.

In short I noticed enough of a difference in my cars to keep me paying a little extra (my oil changes still only cost me about $30). In my opinion it is money well spent, but then again, I only run 91 or 92 octane in all of my cars too.

I will add to this. I like synthetic.
But on the note about leaks, yeah it can happen. Like everyone else said, it cleans the gunk out and makes them leak. I'm not saying yours will, but it's a possibility.
I will also add, on my 89 RS with the factory 305, I changed the oil to synthetic. It always started fine, but after about a week, the valve seal had gotten to the point that it was leaking so much on the spark plugs it almost wouldn't start.
It's not bad oil, it's not bad to switch.
You just need to be ready for a leak or two if you do switch.

I switched my 350 in my 83 to synthetic after the first year (had it for 6yrs now)... took the intake off last spring, and it still looks like its never even been started its so clean inside. Its works very very well. On really hot days, the oil pressure stays normal, where the conventional oil would get fairly low. On really cold days (was a year round driver for a few years, so it saw some -40 and -50 weather) it started alot easier with synthetic than it did with conventional oil.

My LT1 has been using synthetic as long as I've owned it (2 yrs) and supposedly since it was new, if what I heard from past owners is true. I haven't opened it up at all yet, but the oil that comes out at 5-6000kms looks only very slightly darker than it did when I put it in. Its still hard to read on the dipstick at 5000kms, because its so clean. Most cars I've seen that just use conventional oil, especially cars that are 10+ years old, the oil is usually very dark by about 1/2 way through an oil change interval. Not all, but most cars.

oh ok i get it now i didnt believe those rumors but still wanted to make sure and no i dont want to change to synthetic because of price (because i work at autozone either way) but because as you guys said i seen how cars that use it are cleaner than cars that dont as for the motor the old owner used conventional but supposebly changed it before the due date which i hope is true.....i was thinking of either usign castrol synthetic or royal purple any recommendations between the both?

by the way thanks for all the replys! help clear my mind from the doubts

I don't know how to say this tactfully, but except for your statement that synthetic doesn't break down and synthetic blend is cheaper, everything else you say is wrong.
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Absolutely not Castrol.

Royal Purple is the lesser of the betters.

The best choices would be AMSOIL, Mobil 1, Redline. I've heard some not-so-good things about RP lately, but nothing I can substantiate.

Agreed on all points.

wheere could i get AMSOIL or Redline

There is an AMSOIL dealer site sponsor. Or, check my profile.

all that information was given to me by my professor. whos an experienced gm, toyota, bmw tech.

One of the best sources for bad information, unfortunately.

I have regular oil mixed with a quart of Marvels Mystery Oil and ran it for a while then changed over to Royal Purple. So far so good no problems, I have been looking around to see if I have gained any oil leaks but everything is going good with it.

The key to switching over an engine that has been run with petroleum oil to synthetic is cleaning out the deposits left by the petroleum. There are various ways to do that, such as a quart of ATF before changing the oil, solvent flushes (AMSOIL makes a good one), or blends.

If you don't mind frequent changes, you can start at the first oil change by using one quart of synthetic with the rest petroleum, increase that to 2 quarts synthetic the rest petroleum with the next oil/filter change, then 3 quarts, etc., until the change is all synthetic.

Some engines have been so neglected that some disassembly is required. We have a '92 Lumina 3.1 that came into the family with about 140k miles on it. Ran fine, flushed it, put in synthetic. It had a leak at the intake ends because the factory silicone sealer had become rock-hard. In the process of changing the gaskets at about 175k, we discovered extensive hard deposits from neglected oil changes and petroleum oil. We scraped all that stuff out, did another flush just to be sure, and put synthetic back in it. It's still running fine with over 200k on it now. No way those deposits were coming out with flushing or incremental change-over. Even so, in this case the leak wasn't caused by the change over to synthetic, but by the hardening of the silicone sealer from prolonged exposure to heat and the petroleum-based oil.

I've had my 89 Iroc for about 2 years with Mobil 1, 10-30wt., and the oil pan is now leaking at the front seal. Previous owners had conventional oil in there for the first 94K miles, and then switched. Coincidence?

The advantage of petroleum-based oil is the engine will wear out sooner so you can replace all the gaskets and seals when you rebuild it.

i have heard about the ATF fluid but dont quite get how to use it... do i put it a week before the oil change and run it like that or the same day of the oil change or what ?

Bingo. In fact there are some military studies that say you never need to add more than one or two quarts mixed in with petroleum and you still get the benefits of syn. without the cost.

why not castrol? i have always used it without problem. almost thought about mobil1. i run syntech with lucas.

also, why is it that i have good oil pressure then when my car gets hot(225) after driving hard my oil pressure doesnt wanna stay at 55ish like usual? and at a stop light almost drops to 20 then driving off it goes to about 50 or 45again.
if i switch to 20-50 instead of 10-30 will that help?

Just to throw in some more info, I bought a used Vortec 350 off a guy that came out of his 3/4 ton work truck, so it seen some heavy duty use. The motor had 213,000 miles on it. The only reason he bought a new motor, was it had alot of miles on it, and it was running rough. Turns out the exhaust valve seats were shot in the heads.

Now on to the point, when I pulled off those heads, the cylinder walls had NO RIDGE! In fact, the walls looked like they still had the honing marks when it was new! I asked the guy what made his engine last so long..."I ran Mobil 1 in this thing from the very first oil change." Just looking at this engine with my own eyes made me a 100% beleaver in synthetic.

Don't mean to jack the thread, but if I go back to mineral oil after using synthetic, will the seal-blocking sludge build back up again? I ask because my truck (I know, not thirdgen but still a SBC) spits out almost all five litres between oil changes (every 5000km), mostly out the rear main seal. It's definitely not burning any, and didn't leak that badly until I had the truck a year or so and had been running synthetic a while. Never made the connection until now...

Probably not fast enough for you. That sludge build up usually takes 30,000+ miles to build up, and ususally on neglected engines at that.

anybody know about why my car is doing that?

Because oil gets thinner when it gets hot. Completely normal and harmless.

Why not Castrol? Because it isn't a Group IV PAO base. Cheaper, most of the benefits, but not the "good" stuff. Caution, though - even Mobil 1 and AMSOIL have Group III "synthetic" lines.

Having been in the military and worked for a military contractor, I wouldn't put much stock in those military studies. Getting 10k miles out of a deuce & a half engine is considered exceptional longevity. Oils are formulated as a package; mixing brands and/or types is not a good practice.