Viscosity needed is to an extent determined by local temperature, check out any newer glove box book - our hyundai diesel lists 10/30 for milder climates but 10/40 for hotter climates and some really thin stuff if you go below 35f - the tolerances on that particular engine are insane compared to our old stuff, measured in thousands of a mm and really tight, its a 17.3:1 25psi boost deal though

Dom are you still in Nevada? its generally hot there? maybe a 30 for winter and a 40 for summer?

If your running smoke free currently use the thinnest (within reason) that keeps your oil pressure where you like it

Maybe go have a look see over at bobistheoilguy