The answer, which has been confirmed to me by the customer/technical service groups of several major oil producers, is that different oils of similar or better service designations are compatible one with the other.
That includes synthetics and fossilics.
No need to flush.
You can even top up with a different brand if you run short.
I accept the concensus.
Far better than letting the oil get too low while trying to get the same brand you started with.
The API
Heck it all came from the same place, Mother Earth.
True, but I know for sure that there are at least two bases:
Tar Base, and Parafin Base. Quaker State and Penzoil used to be Parafin Based.
Since Parafin is a lot more slippery than tar, I know which kind of oil I like to use! ;)
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