Just a generic comparison of viscosities among a few oils and grades. Higher the number, the more viscous the oil is. You can see that at operating temp, the viscosities arent drastically different. However, at 100* F, it varies a ton. Big difference between straight 50 and 15w50 at 100*, not so much at 210*

Viscosity = the resistance to flow, the reason the multi viscosity oils were developed, I ran straight 50 in a drag car but would not in a boat

I think when we are talking bearing clearance vs oil weights, it assumes we are all running the same oil temps. If we feel say, 20 centistokes id fine at 212*, 6000rpm balls out, why do we need 200 centistokes when idling out of the harbor ?

That chart pretty much sums up my thoughts. Even if you had large clearances, you may consider a 40wt oil if your oil runs cold, or, maybe a 60wt if it runs hot

And more cushion for the pushin

Has anyone used?

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So larger clearances equals viscous oil, adds up. So now what are typical merc clearances?

A bit off topic in regards to oil grades and brands, but I took note of the oil pressure on the 485hp 6.4L in my Challenger. 0w40 Pennzoil synthetic, all readings taken at idle in gear.

60*=55psi
100*=53psi
125*=48psi
150*=42psi
175*=34psi
200*=29psi

I see alot of guys complaining of low oil pressure at idle after a run in the boats. usually they want to go to a heavier weight oil to solve the problem. While it may help, often its because they are simply getting the oil too hot. You take a big block thats custom built, chances are its gonna have fairly loose clearances. When that oil gets hot, as in 220, 230, 240* plus, even a 50wt oil is gonna be pretty dam thin. Thin oil, loose clearances, its gonna be bleeding out internally at idle from the mains. You knock that oil temp down from 230* to say, 180, theres a good chance you'll find 10 or more psi at idle after a hard run.

Agree, changed my oil cooler out 3 times before I got it barely down below 200* (going in) cruising