that is interesting. thank you all.
BTW I do use a centrifuge, and while I am not paying crazy attention to viscosity, it is something I pay attention to. I have 3 diesels. 89 350 crew cab, an 89 250 extended cab, and a 90 GMC Suburban with a 91 Cummins in it. The 250 I do have a fuel pressure gauge in it, and I don't seem to see much difference with pump diesel vs WMO, BUT if it is cold out the truck doesn't like the WMO as well. So far I have mostly just run the WMO in the summer which works fine.
Now I will throw out something that I have not looked into deeply and is maybe crazy- haze anyone considered adapting the second gen Cummins piston pump for the IDI? The 1st gen uses a diaphragm pump similar to the ones used on the IDI. One of the upgrades for the those that have the VE pump on the 1st gen is to switch to the higher flowing piston pump. It requires a spacer and a different spring to lower the pressure to be safe for the VE pump seals. It runs on the cam same as the original.