Volvo 4.3L gas engines, 1993. The manual says to use 15w50 oil but the
dealer says that the manual's outdated and now everyone uses straight
40 weight. I'm in the northeast so water temp and air temp changes
from lows in the 40's to the very warm summers.

Also, Volvo 290 duoprop outdrives, manual says 10w30, dealer says 90
weight?

Now I'm confused.

Any ideas? I'm leaning towards the Volvo manual and use what I've used
for years per the specification.