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
O.K. I'll throw out a couple of thoughts (opinions) while we wait for the more current marine savvy folk to show up.

I see little point in worrying about the xW number for that usage.

I think T6 (also consider M1 0W-40 or TDT) would be fine.

Boat use is fairly heavy duty / high load compared to just driving down the road in a pickup or car. I'd compare it perhaps to towing a moderate load, even if you don't run around at rated maximum RPM.

It might be helpful to know what the typical cruise RPM and rated RPM are and if it has a closed or open cooling system...
Thanks DuckRyder for the thoughts.

-I cruise at pretty much 2600-3000rpm, I never gun it to WOT. WOT for this engine is 4800RPM.

-The cooling system is a raw-water cooled system, thermostat rating for operating temperature on this boat is 160-165. I mostly run in fresh water, sometimes a brackish.

Im mostly convinced even a 5w40 will be fine. I can't see a HDEO like T6, shearing down to anything lower than a 30 weight viscosity after 40 hours of use (my yearly hourly usage). Even at a cold start up in 50+ degrees outside temps I can't imagine the oil is going to be much lower than a 30 weight before it gets to operating temp.
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