Do I use Synthetic or Convetional oil in a 1990 GMC SUBURBAN 5.7 V8?

[M3 Mitch]

Particularly with you being in New Orleans, not much to be gained by switching to synthetic.

You didn't say if this engine leaks or burns any oil to speak of, although I think it's essentially an old-school small block Chevy, if so, these generally leak a little. You didn't say if it's been kept really clean internally, and if you plan to drive it as long as you can or will get rid of it soon. All these would influence if or when I might go to a synthetic - if it's a real creampuff, and you will keep it as long as you can, you *might* possibly get some benefit. But it's iffy even then.

What you might want to do is add one of the ZDDP additives since the current regular oils have less of that than older versions. Jury is out as to if you really need to do this or not - I have seen many older rigs like this that have run fine for a long time on current SM oil.

[texdav]

Originally Posted by Grandpa Pipes

Not 100% true. Syn oils will always super clean the carbon out of the inside of an engine which can lead to one hell'va oil burner! There really is no other reason not to switch to syn oil in a high mile engine but that's more than enough!!!

The only benfit of synthetic is that it handles heat better before jelling. The additive package is what cleans and it wears out same has crude based. That is why manufacturers cal for the same change interval whether crude based or synthetic.

[Carolina Knight]

At 80,000 miles, it is beneficial to use a high mileage or synthetic oil, likely a blend, to combat engine breakdown (leaks, deposits, sludge and friction), lubricate seals and gaskets and provide better protection in cold starts.

Before putting a better oil in the engine, an engine flush would be beneficial to removing deposits and sludge from the interior walls.