Syn 5w40 (Shell T6, Delo) for a gas marine engine

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Jul 26, 2014
Montgomery, NY
This is would be in use for a 1999 Volvo Penta 5.0L GI (Throttle Body Injected EFI)

I know a lot of people that swear by the conventional Rotella 15w40, but I have been running synthetic Straight Weight SAE30 (per manual) since the boat was new (just passed 600hours). Volvo Penta then discontinued the SAE30 a few years ago.There was a lack of clarification of what oil was now suitable for 2007 and prior engines. Per a Service Bulletin issued older gas powered Volvo owners were to just use Volvo's branded 15w40 VDS-3 Diesel oil (which is a non synthetic) after years of recommending only synthetic SAE30,or pure synthetic 20w-50, 15w-50. Frankly I think Volvo will sell what keeps their SKU count the lowest.

I would use mobil's 15w50 synthetic, but I think a 50 weight oil ,even after shear down, on a non-carbureted, would be a little thick for my needs in a 5.0 liter engine that is fuel injected and not run 'hard'. I can see the guys running the big block 7.4/8.1 or top end 5.7's maybe needing it.

Has anyone started using the 5w40 T6 in a marine application?

The 5w rating is the only thing that is giving me pause on using it. I know Merc and Volvo typically like oils that have a closer Cold flow and Operating Temp rating (i.e 20w40), but at the same time isn't the 5w spec irrelevant as I would only be using this in temps above 50F, mostly 70-85F 95% of the time?

Any advice would be great
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I used the 5w-40 syn and 15W-40 dino diesel rated oils in a boat I had years ago. Both were excellent. Prior to that, I've used the 15w-40 diesel oils for as long as I can remember. No way I was going to pay Mercruiser what they wanted for their 25w-40 oil. Never had any issues.
I'm confused why you are brushing past the 15w40? I've started using it is everything except my cars which include mowers, motorcycle, 50hp outboard... anything 4 stroke. Can't say its a good move or not but its simple, easy, seems a lot of others do it and I haven't had any issues yet.
I use the 15W40 Delvac 1300 in my Mercruiser 350 Mag, never going to be that cold that I'll need a 5W in it. I read something about syn oils and moisture in marine applications, but the Mobil is so superior to any gas engine oil that would have been available in '88 when my boat was built that I've never considered using anything else.
RoyalPurple and Amsoil have SAE30 oils.

Stick with full synthetic. Any synth 10w30, 10w40, 5w40, or 0w40 will work fine.
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