Be careful about advice from mechanics and engine builders. A good friend of has been working on cars since the 60s. He says you should never change the
oil or transmission fluid. He says "they get used to each other"! He also smokes like crazy, "when it's your time, it's your time" he tells me.
You can always find a builder who recommends non-detergent, straight weight paraffin
oil. I'll bet the majority of race cars, however, are running on multi-viscosity
synthetic oils. Porsche, Corvette, and others would not use them if they didn't do the job.
Bob