Why 15W40 instead of 10W40 ????

PCM calls for 15W40 oil that is used in diesel engines. Why? These oils don't have the 50 star stamp of approval that other oils have. Would it be the same or better to run Castrol GTX 10W40 rather than the recommended (Dellco) 15W40?
My dad and I disagree on this, and wanted to hear what other think about it.
BTW, I am in a SoCal climate.
Thanks,
Darin

The first qualifier for me would be whether or not the boat is still under warranty. If so, I would use what the manufacturer suggests without question. No need to give a manufacturer a way out of fixing your boat.

If it's an older boat that's out of warranty then I would say you could use whatever weight best fits your climate and/or scenario for how the boat is used. If you are heavily weighting your boat for wakeboarding then I would use the heavier weight for better protection.

'If you are heavily weighting your boat for wakeboarding then I would use the heavier weight for better protection.'
ROFLMAO

And if you are not weighting your boat you don't even need the oil. So just match the weight in your boat to the weight of the oil. Pounds or kilograms it should all work out the same.

Well the Diesel engine oils have some different ratings and such that regular oils do not. I am suspecting that they may use diesel type oils that stand up to oil shearing better and do not lose their viscosity as quickly. I can't say exactly, but I would not deviate from what PCM says. I figure they know what they are doing. Also, these engines are not run like cars engines, much higher RPM's for the most part, which may be the reason for the Diesel rated oils.

I have also heard it said that some mechanics suggest plain old 40wt straight. Again, just something I heard and I will stick with the info in my PCM manual.

Also, being in Socal as well, I run the boat in hot weather in warm lakes, Perhaps PCM figures this in and keeps it at the 15W40. If you ran the boat alot in the winter, than maybe a 5W40 or some such thing like Diesel truck owners will do, but I would assume that the lake would be frozen anyway.

I would stick with what they tell you to use and focus more on keeping the oil changes at proper intervals, that will be the best for your engine no matter what.

Not sure what is so funny T?? If you are weighting your boat, you will be running higher RPM's, which means more strokes, more heat, more shearing, more oil breakdown.

Think about it.

I have some of my info crossed up here, so I may not have it all straight. But I would say stick with the 15W40. i checked a few sites and the consensus was that 10W40 has too many polymers and breaks down easier. The 15W will flow better at cold start I believe, and will not break down as quickly. I do know that oil shear in Diesel engines is a problem and perhaps the PCM people like the diesel oil for it's resistance to it.

You will find many automotive oils have a diesel rating as well as the standard automotive rating. I would stick with the ratings by the manufacturer until you are off warranty but I always change to synthetic as soon as the engine is completely broken in, and then stay with it until you get an oil leak or start burning oil.
The 10W flows better by a bit at cold start. The synthetic flows much better at cold start even if it's a 15W rating.
I think T was just punning about heavy weight in the boat and the heavy weight oil.
I have always run multigrade oils in race engines and had good luck. The straight weight oils don't have as many polymers, as Flux said, so some think they handle load better. My thought is that, since most of your wear is at engine start, you more than make up for any problems by using a multigrade. Just don't buy the cheapest brand you can find unless you change it lots.

Stick with what the manufacturer suggests. The recommended oil from a reputable company will be good enough.

I think what T was laughing about is that there is no difference in viscosity between 10W40 and 15W40 when running at temp. Here's what the ratings mean:

The higher the number, the more viscous the oil is. So, a higher number will generally protect better. The first number is the cold viscosity and the second number is the warm viscosity. So, for 10W40, the oil starts out at 10, to help reduce the amount of "work" the engine needs to do to turn over. When the engine warms up, the viscosity increases to 40 for better protection.

Boats are run in the summer, so they don't need a low, less protective, starting viscosity. Too high viscosity will be hard on a cold engine if you want it to last, so don't use straight 40 weight either.

That's funny because PCM recommends running 40 weight in our 2001 Air Nautique with a GT40. I'll stick to the manufacturer's recommendation as well.

I agree with Art in that you should stick with manf. recommendations. Also, oil lubricates and cleans your engine and how much does it cost to put quality oil in the engine vs. cheap oil and a rebuild.

I would not run car oil in my boat. The additive package is different for boat and diesel engines and stands up better under the high loads. I run 15-40 Pennzoil Marine in my Indmar/GM powered boat since that's what it was born with.

I'm struggling with going over to a synthetic marine oil, don't really know why since I run Mobil 1 in my truck. Probably since I change the oil regularly in my boat and like to go at least 4K in my truck between changes.