Sorry the above current specs should read SN not AN. SN is the current API Spec on most current oil viscosity with the exception of some European Manufacturers.

I swear the Merc manual for the 600's called for 20W50 and specifically referenced Kendall GT-1. ;

Although I didn't enter the conversation over viscosity vs protection (valvetrain stability vs oil I guess does not make sense to someone) I will add some proof on the _w-50 oils being proper for many performance sbc/bbc marine engines.
http://www.mercuryracing.com/blog/vital-fluids/
http://outboardparts1.com/merc/13AG41/264.pdf

Mercury 92-8M0053665 FCW® 25W50 Synthetic Blend (1qt)
Mercury 92-8M0078014 FCW® 25W50 Synthetic Blend (4qt)

All Merc Racing stuff now 25W - 50 the new Merc Racing oil is listed. Back years ago the HP 500 carb app use to have listed on its flame arrestor 40 w for motor oil. Also in Mercruiser service manuals straight 40 w was their second listing for all their marine engines no matter what it was except in temps below 50 degrees.

Next Volvo Penta up till 2 years ago recommended straight 30 w syn motor oil.. Now they have changed to a 10w-40.

OMC sterndrives in their day listed 30w motor oil
Next, 7 marine and PCM and Indmar marine LS 557 hp marine engines recommends 5w-30.
Next, all other Indmar marine engines recommend 15w -40 Pennzoil marine except the 557 hp engine.

And here is one, at one time Mercruiser 55 gallon drums at Stillwater plant - black production engines was 15 w -40 Pennzoil marine even though they recommended their own 25w-40,

I don't see it as proof, I see it as preference. Mercury marine is definitely not the absolute go to for oils(obviously they are buying a product and turning around and relabeling making quite a hefty profit). I think you missed out on the conversation I was having with BUP regarding many conventional 50wt oils shearing because of VII. In this case no wonder they spec a thicker oil. With use in many applications it likely shears to a 40wt.

GM has a good understanding of this issue, especially regarding automatic transmissions. Dexron III for example starts out at a viscosity of about 7.5 and many times ends in the 5 range with use. It's a fluid that just didn't resist shear all that well with use. Fast forward to CAFE requirements and DEXRON VI which is a shear resistant fluid. It starts out at about 5.5 and shears very little even after many miles of abuse. Many were confused at how this fluid could be backwards compatible for vehicles specd for Dexron III... Not only is the friction modification adequate(actually many times better in dex VI) but the lower viscosity threshold protects more than adequately in these applications. If the fluid started at 5.5 and sheared, this wouldn't be the case.

So in summation, not all fluids are comparable in service due to formulation and the presence of VII's. Mercury is all about profit and warranty is part of this. Do you really think they would dump dollars into "the best oil" like a shear resistant Ester to run the exact grade for the specified drain interval? I think not. It's a dollars and cents issue, not always what's superior. Case in point, as BUP mentioned previously, Mercury Marines "synthetic" gear oil... What a joke that was... But they are God what they say is gospel.

http://forum.chaparralboats.com/inde...howtopic=23569

All Mercruiser is going to 25W-50 even the cat engines. I could not post the SB from Merc but found it here. I guess this dealer scanned it.

Next Merc HP 525 lists from OEM Service manual # 6 brought to dealers in June of 2003 -
Merc Racing recommends using a 4 cycle marine engine oil (25w-40) or a good grade multi viscosity 20w-50 automotive oil of the correct API classification would be the second choice but less preferable choice.

The listing in chart form as follows. If not using 25w-40 in which think about it who in the world has 25w-40 motor oil expect made to order Mercruiser. ( By CITGO if I recall correctly)

All temps SAE 20w-50 - SH, SJ,SL,CF-4/CG-4
32 degrees and below. SAE 20 W - SH,SJ,SL,CF-4/CG-4
32 degrees thru 50 degrees SAE 30 w - SH,SJ, SL, CF-4/ CG-4
50 degrees and above to whatever - SAE 40w - SH, SJ, SL, CF-4 / CG-4

Everything currently for Mercruiser has changed and if temps are 50 degrees and above they recommend their newer oil 25w-50 syn blend.

The proof is their engines dont blow up!!!! Go try and talk a guy who just dropped 100k on a pair of merc engines into running an oil thats been unproven to work in them, and simply based on your oil research. Good lord.we get it, you dont like 50 grade oils.

Yup gonna use that new stuff next time.

Don't mean to change the subject, but. "The proof is their engines dont blow up"!!!! SERIOUSLY Merc engines don't blow up. What a crock.

Did I say it was going to blow to?? Plenty of them do, however that is a separate topic. Mercury marine isn't the engineering powerhouse you think they are.. Jesus you need to read my posts. Were you absent for the explanation of likely shearing that exists in oils with high VII? You always have to get the last word don't you...

Plenty of guys use oil in other forms of Motorsport that tax oils as much if not more than a boat. You act like marine endurance is the epitope of oil abuse. These engines are every bit as expensive, and I can tell you they surely aren't running the OEM high Performace recommendation...