The synthetics are fine for a modern motor, they were designed to use them. But our older motors were designed for Dino oil, at a certain thickness, a certain viscosity, certain flow characteristics, with certain additives. As pointed out by hotrod Lincoln, the thinner full synthetics find the path of least resistance and have a higher tendency to leak, think that pesky rope seal, don’t provide the appropriate amount of cushioning, and thus may not be the very best choice.
There is one alternative, brad Penn, now called penn grade 1, is a Dino based oil, with the zinc for the cam, and a synthetic additive package. Termed a semi synthetic. I run that stuff in my 235 and 261 motors and have done so for quite a few years, and I am not easy on my motors. Not a single hiccup on any motor I have used it in. They also make what I think is a terrific break in oil.
Just an alternative.
