[v8volvo]

The myth of different oil types causing or solving leaks is mostly a myth.

If there is a gasket that is already failing, then I suppose running a thinner or more slippery oil could increase a leak.... and on the flip side of the coin, yes there are "seal conditioner" products out there that promise to give new life to tired/brittle seals too, or thicker/worse oils.

But remember the main purpose of the oil is to lubricate and cool internal parts of the engine, not limit leaks from seals. That's the seals' job! Best choice is to use the right oil for the engine, and deal with leaks by making repairs, rather than attempting a fix in a bottle. You don't want to sacrifice anything in the primary purpose of the oil by chasing after leak mitigation, which is what you would do if you used mineral 15W40 rather than a good synthetic 5W40 or 0W40 (or some like 0W30/5W30 if you live where the weather never gets hot).

Your D24 should also be happy with the 10W40 Motul in the summertime, as long as it is rated for diesel engine operation of course. In the winter you want a 0W or 5W weight.