I received a PM...guy wants to go with a straight weight oil in a tricked out 700 HP engine...ambient temps are 85-90* F.

I have no suggestions for a straight weight...because I would not go that route personally.

My choice would be Valvoline 20W50 VR-1 Racing Oil...you have high pressure springs on those flat tappets and this oil has a ton of ZDDP. Faster start-ups are good over a straight weight. Excellent friction modifiers as well. It's a "balanced oil". ONe of the best of it's kind. My choice for offshore engines if you want a conventional. Synthetic? M-1 V-Twin.

Get a Wix/Baldwin/K&N or NAPA Gold oil filter and you wil be all set.

http://www.valvoline.com/products/co...ng-motor-oil/6

I don't know the temperatures that the low viscosity of the oil kicks in at, but I was under the impression that they're below the ambient temps that you're suggesting.

I would run a synthetic straight 50 weight after break in.

merc uses a 20/40 but for yrs I thought it was a 40, I have used a 40 and a 50 with no troubles after doing mods to motors. I heard of guys using m 1 syn 15/50 which I USED on my 650hp motor.

My buddy and I run straight 40 from NAPA. Mine 625 his 700hp. Never had a problem, change every 20hrs and looks fresh coming out...

I have similar hp. Engine builder still services engines and won't use anything other than Valvoline 40wt. racing oil. Wix filter.