Then I'd take that question one step further and ask, why not Amsoil TRO (20w-50 Racing/Severe Service)?

My quesiton would come from my perspective in my 540ci application. I ran Valvoline 50w Racing during the first 40 hours but that stuff is THICK at start up. With either one, I could probably free up 50HP lost in parasitic drag with the Valvoline!

Seems like a 50 weight oil is not a good idea. With the motor cold it's likely that 1/ the oil pump bypass is open reducing flow and/or 2/ the block bypass will open allowing oil to bypass the filter and/or 3/ the oil filter bypass will open not allowing the filter to do it's job. the 50w is measured at 210*F so you can imagine what that stuff is like at 60*F.

The main reason for not running racing oils is that they generally lack detergent properties and require frequent changes, like every time out.

The m-1 v-twin 20/50 seems to have the best of all properties for our boating needs. In fact we have started running it in our Late model and UMP Modified dirt race cars also.

My engine builder recommends straight 30w Valvoline for break in.

For auto or marine or both? Seems like there would be a difference based on clearances between a motor built for the street and one built for a boat.

I believe that as far as marine/auto clearances go, most bearing clearances are the same. Some allow more piston clearance in some marine applications. I believe that GM does not.

My builders break in oil choice is his personal preference, he also uses straight 30W Kendell if the Valvoline is not available.

Obviously you have to go with what your builder recommends. Just surprised at the straight 50W, something I would expect to see in an older Harley

Wobble, there is no such oil viscosity as 50w. The correct viscosity is SAE 50. The "W" in the viscosity desination means winter, not weight. The winter classification of an oil is at 0 deegrees F. So a multi vis 20w50 meets a 20w spec at 0 degrees F and an SAE 50 at 210. A 20w oil at 0 F is a much heavier fluid than a SAE 50 at 210. Remember all oils thin with heat. Oil viscosity does not go 10,20,30,40,50,60,70. Ther are 0w,5w,10w,15w,20w,25w, all measured at 0 degrees and SAE 20, SAE30, SAE40, SAE50. 60 and 70 viscosities are not reconized by the SAE. These viscosities are refered to as Grade 60 and Grade 70.
Ken

I stand corrected, I did mean SAE 50 or SAE 30.

The reason I only included the ARO and MCV 20W-50 oils from Amsoil is that I was not sure about the anti rust/corrosion properties of the TRO oil. It seems that the MCV (Motorcycle) oil has been tested against ASTM D-1748 (humidity cabinet rust test), but I am not sure about the difference between the ARO, MCV and TRO oils generally speaking.
Anyone who knows which of the ARO, MCV and TRO 20W-50 oils from Amsoil would be the best for a marine engine ?
Thanks.

Since Amsoil seems to recommend anything they are asked about, (I think I could ask about silicone spray being used in my 540 and they'd agree that it would be a good choice ) I tried to find Royal Purple Racing 51 and no one that sells RP around here stocks it, I ordered Amsoil TRO 20w-50 Racing.

I'll see how it does. What I'm hoping for is less drag on cold start, maintain good oil pressure (I think OP was a little to high with Valvoline 50 Racing) and gain 150HP rescued from drag. LOL Ok, better cold starts and good oil pressure and protection would be good. I'll be changing it 2-3 time this season anyhow.

Mobil-1 20W-50 V-Twin is a tough cookie to beat and once you use it and discover how good it really is ...you get addicted to it because it offers a superior level of protection that nothing else can match let alone beat in the marine environment. The heat protection...anti-corrosion and rust protection...dry-start protection for storage up to 4 months...tons of all the right additives...the perfect oil pressure and temperature...it gets the job done like nothing else.

So Wobble..what oil filters are you using anyways?

Hydro, If you are only putting about 30 hrs per year, N/A motors, indoor stored. How often would you change the V-twin oil. Would it be acceptable to use late May through September? I have decided to use this oil. What's your opinion? Eric