I have never had a problem running synthetic oil in older motors. I have Mobil 1 in my all original, never been touched 73 Z-28 and it is fine. Although I am a firm believer in synthetic oil, I use Mobile 1 in everything I own, I personally think that Amsoil is a bit overpriced. Just go to WalMart and get Mobil 1 or Castrol Syntec etc and save the hassle.
Advanced Automotive Machine
1971 Chevelle "Heavy Chevy" original owner
1973 Z-28 ..one family car...Brother bought it new in 73
Just replace with synthetic when you do your oil change. They're compatible. I also use Mobil 1 although I just put Amsoil 2000 gear lube in the differential. I use Mobil 1 in everything, including my lawn mowers - good heat protection.
69 RS Conv,355,M20,4.10's
<BLOCKQUOTE>quote:</font><HR>Originally posted by matth: If an engine has been run exclusively on regular oil, what effect might the synthetic have on it? (about 25k rebuilt miles..)
Also, is the synthetic more prone to leak past seals than the conventional oil? thanks<HR></BLOCKQUOTE>
Effects will include less engine wear, longer engine life, less deposits, less oil consumption, possibly a touch better fuel mileage, and maybe a couple of hp. I switched my 3.8L Buick motor over to Mobil 1 at 45,000 miles. That was 10 yrs and 200,000 miles ago. Inside the valve covers (which have never been off) the rockers and springs look clean and brand new. Motor has never needed to have even one extra drop of oil added in between the 6 to 8 thousand mile oil changes.
As far as seal leakage...supposedly some early synthetics didn't cause seals and gaskets to swell as much as the conventional oils of that time. This did lead to some increased leakage in some engines. But that was long ago and, from what I have read, is not a problem with modern day synthetic formulations.
I have changed over several high mileage engines to Amsoil with no problems. You should use Amsoil Engine Flush first to get most of the deposits out of the engine. Amsoil does cause the seals to swell and doesn't cause leaks. It is a little better than Mobile 1, but Mobile 1 is a very good synthetic oil. The other synthetic pretenders are not much better than petrolium oil.
'69 RS/SS396 pro street
High Performance Drive Train Parts And Service www.lubedealer.com/biggearhead
I had and engine that sat for Ten years. It was an original 302. It always ran hot. I changed to Mobil 1 and I did notice a definate difference in the temperature. I had the engine flushed first on one of those oil flush machines and added the synth. You should have seen the sludge that came out of that motor. I guess that is why they use it exclusively on the new Vettes.
Only 69 Z/28s
Garnet Red 30,000 mi
Rally Green new in 69
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