Anybody using multi-weight oil *must* be very religious about changing it when
due.

That 5w-50 oil?

The basic oil in that jug is 5w. A group of "viscosity enhancers" allows the
oil to provide the same protection as 50w when the engine heats up.

Caution: The viscosity enhancers will break down after a while and the oil you
think is 5w-50 is then really only 5w.

I use Delo 400, 30 weight, in my engine.
Got about 4000 hours out of the first engine, and when it failed it wasn't due
to a lube or fuel problem.

( I have used Delo 100 for the first couple of oil changes on the new engine).