Rotella T6 synthetic oil

I've been using Rotella T5 15w-40w synthetic blend in my 2001 8.1s, works great, pressure is more stable than the 10w-30w oil that was in them when I bought the boat. I'm thinking of going to Rotella T6 5w-40w full synthetic, any comments. Crusader says synthetic is okay to use, just wondering about the viscosity.

I'd be inclined to stick with a 15W-40 synthetic.....when in doubt, stick with the recommendations in your owner's manual....

15w-40 full synthetic is not available. Checking further I find that Rotella states the T6 5w-40 full synthetic is a direct replacement for regular 15w-40.

slow cruiser said:

.... Rotella states the T6 5w-40 full synthetic is a direct replacement for regular 15w-40.

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That may be their opinion though there's no logic supporting it.....the required viscosity is called out in the owner's manual.....if you want to deviate from it, that's you choice....

Well the owners manual is from 2001, almost 20 years ago. Oils have changed/improved/declined considerably since then. EPA regs. regarding zinc and other heavy metals have changed. So what the owners manual wanted in 2001 and what is available and should be used now is in considerable variance, IMHO. The Rotella oils have dropped the amount of zinc, improved friction modifiers have made a wider latitude of oil weights available, and if Rotella says 5-40 is a direct replacement, I would say go for it, if you want to use Rotella.