GM has been recommending 5w-30 since the mid 80's, and 10w-30 since the 70's.


Here's some info on mulitgrade Oil -- My father in law owned a Lincoln
mercury dealership for over 30 yrs. When I first started working for
him, part time. I watched as 30 new Lincolns came to the car lot via
tractor/trailer and getting unloaded. My father in law, stated. Joe,
drop the oil on each car, and fill with 20/50 oil (this was in the yr 1974)
I asked why ? is not the oil from the ford factory any good ??? he stated
yes the oil in them is good, (first reason) but if you want a quiet
Lincoln, you run 20/50 oil, (the second reason) my father in law, also
stated, from repairing engines, 20/50 in most climates will make the engine
run smoother, longer service, and less repair,other than (extremes like
Alaska) or South America

Now we notice, 30 years later (2004) most oil companies list 20/50 for
racing and extreame usage of a engine. I guess this info would also apply
to inboards