If you're going to use synthetic oil, you have to start with a relatively new engine. After a few thousand miles, mineral oil causes the seals & gaskets to swell. As long as mineral oil is used, the seals/ gaskets are tight. If you switch to synthetic, the seals & gaskets shrink and leaks develop.

Some truth in all of the above. Using synth oil is good. But as stated before, worn, dry gaskets and seals that did not leak with standard oil will leak with synth. If the gasket and seals are otherwise good, they won't leak when you change over. That simple.
Synth will cut friction, but that depends on the machining that was done on the engine, most older engines won't notice a difference.
Synth will last longer before it breaks down, but the major cause for most of our oil changes is contamination, not breakdown. Synth will become contaminated just as fast and as bad as standard oil. So no advantage there for most of us.
If you are racing or subjecting your engine to severe use, the synth will provide more protection. The way most of us drive standard oil will do more than we need done. This includes the short burst of power we are inclined to use, and a good haul down the highway. We are not normally at Wide Open Throtte for more than a few seconds to a minute at a time. There we don't need the extra of synth.
Many new cars have smaller bearing and contact surfaces, or have much more stress due to greater engine output. These need a stronger oil film provided by synth. If you have built up your engine to 400 hp or more you may want synth.
If you have a high mileage engine with some wear, you don't want synth, it flows too easy and may drop some oil pressure through larger clearances. Other than this or leakage on some engines, Synth is a better oil, but more money and cost than you may need. Consider all the factors.

If you have a fresh engine and want to use Synth, run standard oil the first oil change to help break in the engine, unless it's a new design and/or high HP, then switch to the synth at the first change.

I use Exxon Superflow 10w30 in my C3, Celebrity, and Caravan. I also use Justice Brothers Engine Oil Treatment about every 30,000 miles. It treats the metal, not the oil, and stays there after an oil change. Can't get JB? use Slick 50. The others are not as good, and may even do some harm.
30 years, 24 ASE certifications, 90 tech articles in trade magazines, teaching at a Tech College, behind me and personal observation brought me to this choice. It may not be right for you. There are many variables.

Feel free to ask me. Give me your details, and I will help you choose where I can.

Ken Styer

Look at it this way:We all have old C3's here that were NEVER designed to run on synthetic. Those old engines dont have the close tolerances of todays precision machined powerplants. Unless you're balanced and blueprinted and spinning 6000 plus quite often I'd stay with whatever chevy calls for and only go heavier depending on climate and/or the mileage on the engine.

Use mobil 1 in my 69 big block. Also use in all my other vehicles. Have good luck with it. Extend oil changes to
5000 miles and change filter at 2500 miles.

john

one big difference between regular oil and synthetic is that synthetic has no carbon in it and regular oil does. as the regular is run and breaks down the carbon will still be there doing its damage. i have worked in Sunoco, Mobil, BP, Arco, Gulf refineries. we used to take regular oil up to its breakdown point, let it cool down over night and then bring it up to its breakdown point again , but the oil would flash at about half its break down point the second time because of the additives in it. this would not happen to synthetic

Very true. Synth oil is superior, most of the time. But most of us never get the oil anywhere near the breakdown point. That's why I recommend if you are racing or sustaining high speeds, use synth. But an older worn engine I would not due to increased clearances vs a free flowing oil.
It all depends on what you have, and what you are doing with it.

This thread appears in just about every forum that is somehow related to internal combustion engines. In reading a ton of literature, some good, some questionable, the prime reason I use syththetic oil in my engines is it's supposed superior anti-drain down characteristics. Supposedly, for the everyday driver, the majority of engine wear occurs on startup, when oil is scarcearound the rings, cam lobes, rod & main bearings. Syth. oil supposedly adhers to the metal surfaces better than dino oil. That was enough for me. ( If it is indeed true).

Excuse the copius use of the word "supposedly", but if I haven't run the tests, I don't blindly accept the results as claimed.

Just my $.02.

OOWV


BB NJ Ken

I whole heartedly agree with After Shark!

I used Mobil 1 religiously in my '69, but after talking with a mechanic who builds racing engines, listeneing to his test of various oils, and looking at some bearings he tested, I switched to Royal Purple. It quieted things down imediately.

Amsoil synthetic is the "life blood" in all my vehicles. Just change the filter every 7500 miles and top it off. Dump the oil every 25,000 miles or send in a sample of the oil for $20 to see if it can be run for a extended period of time. All guaranteed by Amsoil.
Sarge

I use Castrol Synthetic in my 68 vett, 327 -350 hp, with no problems. The car has 219,000 miles on it. I'm the original owner. The only leak I get is at the oil filter if it is not tightned good.