[RalphVa]

I plan to buy a turbo diesel VW Jetta. Will definitely use synthetic oil in it, probably the 0w30 Mobil 1 (this is what Mercedes uses). If you go for longer intervals, the Mobil 1 5w40 "SUV" (and diesel) oil is probably better.

15w40 is too heavy for the winter. You need something 0wxx. In Imperial Research tests, the ONLY oil that would not smoke the upper valve train in winter starts was a 0wxx. In Sun Oil tests, synthetic oils gave NO upper valve train wear; whereas, dino oils did (on taxi cabs). Why? Because the synthetic oil with lower (don't know whether it was 5wxx or 0wxx in their test) cold start numbers are "grabbed" quicker by the oil pump and get up to the valve train on startup.

ANY oil will be more viscous than its later viscosity reading, e.g. its yywxx "xx" value, on startup. So, even a 0w40 oil will be more viscous than a 40w oil on startup.

You guys can be killing your diesel engines by thinking you need 40w in them. I use 0w30 in mine, both the tractor and Benz diesel.

Ralph