JR North wrote:

I have run straight 30wt Castrol in my 5.0 Merc for 15 years. Reason:
the oil temp guage I installed in my '70 F250 390 engine always reads
320°-340° just tooling around town at low rpm. Imagine what the oil temp
is in your 350 at 80% WOT. This is much higher oil temp than cars
usually run. This can break down multi grade oils. Straight grade oils
are resistant to high temp breakdown better than multi grade.
JR
I'm not an oil engineer, I just play one on TV

The cost of synthetic should not be an issue... how much does a marine
engine cost? Just go with Mobil 1 multi and be done. I use 5W-30 &
15W-40 with nary a problem. 1973 Ford 302 inboard. Purolator Pure One
or Motorcraft Filters.

Rob