[Hib Halverson]

For 1990-1992, the factory fill oil for an LT5 engine was a 10W30 petroleum-based oil (not synthetic) and Mobil Oil was the supplier.

For 1993-1995 the factory fill in an LT5 was a 5W30 synthetic "Mobil 1".

A 5W30 synthetic can be used in the early engine with no ill effects.

In fact, you don't even need M1 "High Milage" because the LT5, by virtue of its OHC valve train, doesn't require additional ZDDP.

If I still owned my ZR-1 what would I put in it? Gibbs Driven LS30 which is a 5W30 with a more advanced base stock than any of the Mobil 1 products use.

For frequent track use, I'd go with Gibbs Driven DT40 a 5W40 which uses the same mPAO base stock.