Originally Posted by Ludwig II
Provided they're both mineral oils I see no problem, synthetic and mineral don't cohabit well from what I've read.
I'd like a second opinion from someone.
It is not the base oil but the additive package that creates the problem.
I have had sudden fouling from mixed mineral oils several times in the past.
You are right about marine oils being totally unsuited for our engines.
Speaking of marine, just got back from 2 weeks at sea. The tender's Yamaha 25 outboard gave us headaches from 4 month old stale fuel. No idle and missing at power.