Technologic80 wrote:
This year I am using..... <gasp> full synthetic Shell Rotella T6 5w/40 in my twin 4.3 engines

I'd love to see an oil analysis report targeting viscosity if you're pouring a 5w40 in there!
Oils with a large viscosity range use a large amount of VI Improvers, which shear at a faster rate.
This is why Mercruiser still use their 25w40.
I'd personally never use anything thinner than a 15w40 in any marine application.
It's still >30c down in Australia & I'm currently using a high ZDDP, 20w60!

I researched it ad nauseum, spent a lot of time reading on Bob is the Oil Guy forums...many UOA's showing it is extremely shear resistant. A lot of people using it in their diesel engines, hot rods, and boat engines because of this. Normally I wouldn't go lower than a 10w40 (exception: my one run with HD diesel 10w30)...