I suggest running 10-40 w oil... . Remember... multi viscosity oil stays the same thickness whether hot or cold... but the thinner the oil.. the better the flow... it doesn't matter how much pressure you have... if the oil isn't flowing.. you aren't lubricating... thinner oil passes through the filter easier with much less going around the filter through the bypass. Thinner oil also cools the engine better... the oil flows out past the edges of the rod bearings and flings up to the undersides of the pistons.. where it removes some of the heat from the piston... if the oil is thick.. it isn't flowing as much and removing the heat from the pistons. Hence.. more wear... more chance of detonation.. etc etc. Also ... thinner oil pumps better up the push rods and flows more oil over the valve springs to remove heat there.

Audacity,
How do you like the purple and how long have you been running it? What weight are you using in the engine?
FYI, I purchased my boat from Rick which purchased your boat.

we have been using the purple products from about 95ish on in the drives w/o question.

straight-grade or 41/51 depending on the engine application.

the 75/90 can be used in 95% of drive aps.

RoyalPurple in my 250 PowerStroke,Hp500efi,XZ drive & race car.Great stuff!BOB

Audacity,
Why do you say that the purple can be used in 95% of the drive units why not all?
I'm running a Bravo 1.

"the 75/90" in 95% of all drive applications...they manufacture MANY products to cover all your needs.

Troutly,

I've been using Amsoil 15W-40 for some time now. At my next change, I'm going to switch to Amsoil Series 2000 20W-50 Severe Service Racing Oil. I think it's a little bit better (definitely more expensive at $8 vs $5 per qt), but it may be overkill.

I'm also using Amsoil Series 2000 75W-90 in the Bravo. It works great. Shifts much better than the Merc stuff.

358 sbc 375hp using 15-50 Mobil 1

Alpha 1 with Simrek drive shower using Redline Shockproof heavy

BH

Amsoil Marine and diesel in the engine and the PSD F350. Amsoil 2000 in the Bravo.
I put the amsoil in at the begining of the season last year. 1 drive had to be pulled to put a new shift cable on. The service center changed the oil to Merc hp. Later in the summer the clutch cones welded up in the drive with the Merc oil in it. I wonder if it would've happened if it had the Amsoil in it??? The other drive was perfect all summer.

Rick here is the thread on oil
 

Here is the thread on the oil which I was talking about.