[v8volvo]

I do pretty much the same thing. The oil I use in everything is 0W40 Rotella T6, sometimes 5W40. Any good diesel rated 5W40 synthetic should be a great choice, the ones Anders named, or Rotella, or some folks like Mobil 1 Delvac or TDT. There are probably others. All are pretty reasonably priced at this point and easy to find, and with such a wide viscosity spread you can run them year round.

The only reason I choose the Rotella T6 is because the 0W40 grade is commonly available in it, whereas all the others are usually only seen as a 5W40. 5W40 will cover almost any normal situation, so no real need for the 0W40 most of the time, but on the coldest days of winter it lets the engine spin a little faster on the starter. I have started mine (with great difficulty) unplugged down to about -35F at 5000' altitude. In that situation every single RPM of cranking speed helps. But in general there is no other advantage vs the 5W40.

Conventional oils are worse in every respect, they are not much cheaper and don't protect the engine nearly as well and need to be changed more often. Good ones like Delo 400 or Rotella T4 will do a decent job but there's just no real argument in favor of them, they have no advantage, not even cost, since shorter drain intervals are required. Synthetics perform better in both cold AND hot temperature extremes, have better film strength, and do a much better job controlling deposits.

I would stick with the synthetic but switch from your 10W30 to 5W40.

Another point: synthetic 5W40 is exactly what Volkswagen calls for in more modern TDI engines, which are mechanically identical to the D24 and D24T. No doubt they would have called for it for the D24 family too if good synthetics had been widely available back when the engines were first built.