Vortec Indmar 8.1L (boat) oil suggestions? 40WT

[ringmaster]

My wake surf boat has a 2007 GM Vortec Indmar 8.1L L18 (~300hrs on it) that was originally spec'd for 15W40 oil but Indmar later amended the spec to straight 40WT. I contacted Indmar asking which synthetic oil they recommend and their answer was "We don't recommend using synthetic oil in that engine." I can't imagine why a high-quality multi-grade synthetic wouldn't provide better protection than a basic dino 40WT.

The boat is used in fresh water in ambient temps between 70F-100F and I am religious about never exceeding 2000rpm or adding load to the engine until it reaches operating temp. The oil will be changed once a year after ~40hrs of use which is mostly wake surfing at ~3000rpm and high throttle opening/load.

I am considering a number of oil options but would appreciate feedback and suggestions.

Shell Rotella T6 15W40
Redline 10W40
Redline 40WT Racing (15W40)
Mobil 1 15W50

 

[blufeb95]

I'd use the Rotella T6 out of those, I'd absolutely not use Redline Racing Oil.

 

[JHZR2]

I can't imagine why a high-quality multi-grade synthetic wouldn't provide better protection than a basic dino 40WT.

Better protection by what metric that matters?

Subzero pumpability? Extreme high temperature? Extended drains? Oxidation resistance?

One might argue that for your needs such things are irrelevant.

In the past it had been said that Dino oils deal with water better. I don’t know that this has changed. In your use conditions and temperatures this may be a useful attribute.

Selection of viscosity is usually engine design and condition related. A fair weather pleasure craft sure could be a candidate for a straight weight oil. Here is Daimler’s recommendations:

46EDA1C1-0D61-4712-83D9-CFFEA6BE42C7.jpeg

• JHZR2
• Mar 28, 2021

You can see there’s a functional equivalence ambient condition set. But the allowable temperature band for a 40wt is pretty limited and really oriented to hot conditions.

I’d personally run the 15w-40 but I don’t know that I’d be that concerned with synthetics.

 

[football]

The manufacturer told you to use SAE40.

40 weight is PERFECT for this.

Don’t be scared of SAE40 Dino oil. I’ve used it in generators and mowers and you can’t beat its performance.

 

[wpod]

The engine won't care which one of the oils on the list you use. Most commercial marine operators use 15W-40 HDEO oil in the gas engines and High speed Diesels. I liked big block Chevy engines in my boats and used 20W -50 car oil or 15W-40 HDEO. The 15w-40 HDEO oils are probably the best off the shelf oil you can buy.

 

[ringmaster]

Thanks for the replies. Can anyone think of a reason why 40WT Dino oil would be better than Rotella 15W40 synthetic for this application?

 

[DuckRyder]

In absence of some stated reason in the backdate specification my guess would be that the absence of viscosity improvers is the reason for a non-multigrade recommendation.

Personally if i was sticking to manufacturers recs id probably run some sort of "hot rod" synthetic 40 weight. If i was feeling adventurous either 0/40 M1 or Mobil 1 v twin.

Is T6 still rated for gas engines?

Edited to add: 8.1 is a just a marinized Big Block Chevy right? And @UncleDave

 

[PontiacHO]

Old news but my last inboard Volvo Penta inboard (351 Ford) specified 40 weight oil. I asked the dealer that sold me the boat also said “do not use anything but 40 weight oil”. They used Union 76 40 weight racing oil across their line of boats. They’ve been in business for 50 years so I trusted their knowledge…

 

[Donald]

I think a quality 15W40 would be fine. A gas rated 15W40 diesel oil like Shell Rotella T would be a good choice.

I think the issue they were trying to avoid was older 10W40 oil.

 

[ringmaster]

...

Edited to add: 8.1 is a just a marinized Big Block Chevy right? And @UncleDave

@DuckRyder yes, as far as I know my marine 8.1 is mechanically the same as the 8.1L Vortec in GM trucks.

I can understand Indmar wanting to avoid old 10W40 and 15W40 dino oils with lots of viscosity improvers but surely we've moved past that with high-quality multi-grade synthetics.

My search for synthetic 40WT oils hasn't returned any that are easy to purchase locally. Royal Purple SAE 40 High Performance Synthetic Motor Oil, PennGrade 1 High Performance Oil SAE 40 and Valvoline VR1 Racing SAE 40 High Performance High Zinc Motor Oil.

Still no support for my idea of Mobil 1 15W50 in this engine?

 

[Zap1967]

We have mark 4 big blocks we run in our barefoot boats. They don’t have fancy stuff like the roller cam the 8.1 is going to have so we run an oil with adequate ZDDP.

I’ve had excellent luck as have many others running conventional VR1 20-50. Some day I’ll pull a sample for a UOA but I haven’t yet. These engines see mostly 4000-5000 rpm operating when working and not cruising. Once or twice a season we run wide open throttle for an hour.

This current oil change has one of those race sessions on it so I’ll try and get it analyzed. My engine has never been rebuilt and has around 1200 hours on it currently. We run big oil coolers and my post cooler temps peak around 190-210°F.

The 8.1 is pretty robust so I’d say any synthetic well known 15-40 HDEO would do the trick. I’ve ran 15-40 Rotella conventional in my marine 351s for nearly 20 years with great results. If you want to step up some guys also run the Mobil 15-50 for the synthetic oil and ZDDP content (flat tappet not roller).

 

[oilboy123]

I would use a full syn multi weight oil, like a 15w-40, and not think twice. I think the no synthetic straight 40 weight is from the dark ages.....
I would assume they are afraid of shearing. Guy reccomending it is probably older than me, and does not believe in synthetics.
I Just turned 62

I remember installing a camshaft on a small block Chev in the late 80s, when I had been running Mobil 1 for a few years. It said not to use synthetics. I was thinking that is not going to happen........

 

[addyguy]

T6 15W-40 would be a great oil for this application.

 

[ringmaster]

T6 15W-40 would be a great oil for this application.

Thanks for keeping this thread alive. I like your suggestion of Rotella T6 15W-40 and I can't imagine why the 8.1L wouldn't be very healthy and happy living with this oil.

For the time being I did the winterization oil change with Valvoline VR1 Racing SAE 40 High Performance High Zinc Motor Oil so unless someone scares me away from using it this is what the engine will run for summer 2024. After that I might revisit moving to a high quality full synthetic.

 

[oilboy123]

Racing oil would not be my first choice....... I believe the additive package is more optimized for shorter change intervals.

 

[FranklinJL]

Redline 5w40 is perfect for this. HTHS of 4.4 with a crazy amount of ZDDP. Thicker film that a 20w50 at temp

 

[PontiacHO]

The manufacturer told you to use SAE40.

40 weight is PERFECT for this.

Don’t be scared of SAE40 Dino oil. I’ve used it in generators and mowers and you can’t beat its performance.

My last 5.8 Volvo inboard got SAE40 as per the manufacturer’s specification and dealer’s recommendation. The service manager at the dealer had a huge shop right on the water that was adjoined with their full service marina with wet slips and dry storage. He was factory trained by major manufacturers. I respected his knowledge and experience.

 

[alarmguy]

My wake surf boat has a 2007 GM Vortec Indmar 8.1L L18 (~300hrs on it) that was originally spec'd for 15W40 oil but Indmar later amended the spec to straight 40WT. I contacted Indmar asking which synthetic oil they recommend and their answer was "We don't recommend using synthetic oil in that engine." I can't imagine why a high-quality multi-grade synthetic wouldn't provide better protection than a basic dino 40WT.

The boat is used in fresh water in ambient temps between 70F-100F and I am religious about never exceeding 2000rpm or adding load to the engine until it reaches operating temp. The oil will be changed once a year after ~40hrs of use which is mostly wake surfing at ~3000rpm and high throttle opening/load.

I am considering a number of oil options but would appreciate feedback and suggestions.

Shell Rotella T6 15W40
Redline 10W40
Redline 40WT Racing (15W40)
Mobil 1 15W50

Because a high multi grade synthetic oil have the potential to shear down to a 25 or 30 whereas a 40wt anything will hold at 40 or close to it.

So what would you rather have circulating around the bearings and all the other associated internal components of the engine when the oil is cooking under stress?

The marine engine manufacturers know what they’re talking about unfortunately homeowners with automobiles equate the same scenarios as they do their boat.
The load of a log, which is your boat being pushed through water by an engine is under an incredible amount of load compared to an automobile with four tires turning on wheels and bearings.

Another misnomer is the same example as above people equate oil temperature with water and air temperature. This is not an automobile. This is a marine engine with incredible stresses The internal friction heats up oil temperature is much greater than you may think. Your oil is nowhere near the temperature of the thermostat on your boat engine and the outside air temperature makes almost absolutely no difference.

Hope this helps

Ps the 40 weight Valvoline sounds OK, it’s certainly better than any 5,10,15/40 which is a possible recipe for disaster. Maybe with a 15/50 you would have a chance but 40 straight gets the gold.
One other thing to keep in mind, just to keep in mind, I’m not saying it too negatively, but the more additives an oil has in it the more chances for piston ring deposits in that cooking, hot, marine engine at times.
Additives are good, but don’t think because some oil has an overabundance of them that it is better. Also keep in mind many marine specific oils have rust inhibitors in them, which will help prevent light rusting of the cylinders when the engine is not being used.

BTW Mercury Marine Quicksilver 25/40 oil is a straight 40 (no viscosity improver) and another good option.

 

[OVERKILL]

Because a high multi grade synthetic oil have the potential to shear down to a 25 or 30 whereas a 40wt anything will hold at 40 or close to it.

So what would you rather have circulating around the bearings and all the other associated internal components of the engine when the oil is cooking under stress?

The marine engine manufacturers know what they’re talking about unfortunately homeowners with automobiles equate the same scenarios as they do their boat.
The load of a log, which is your boat being pushed through water by an engine is under an incredible amount of load compared to an automobile with four tires turning on wheels and bearings.

Another misnomer is the same example as above people equate oil temperature with water and air temperature. This is not an automobile. This is a marine engine with incredible stresses The internal friction heats up oil temperature is much greater than you may think. Your oil is nowhere near the temperature of the thermostat on your boat engine and the outside air temperature makes almost absolutely no difference.

Hope this helps

Ps the 40 weight Valvoline sounds OK, it’s certainly better than any 5,10,15/40 which is a possible recipe for disaster. Maybe with a 15/50 you would have a chance but 40 straight gets the gold.
One other thing to keep in mind, just to keep in mind, I’m not saying it too negatively, but the more additives an oil has in it the more chances for piston ring deposits in that cooking, hot, marine engine at times.
Additives are good, but don’t think because some oil has an overabundance of them that it is better. Also keep in mind many marine specific oils have rust inhibitors in them, which will help prevent light rusting of the cylinders when the engine is not being used.

BTW Mercury Marine Quicksilver 25/40 oil is a straight 40 (no viscosity improver) and another good option.

As is typically the case, the recommendations vary wildly depending on the year and model of the engine in question. I've posted these before:

Look how many of them recommend just an ILSAC 5W-30, including both GDI engines, which have higher power density than the two MPI engines that spec 15W-40 and 15W-50 respectively.