hard start, long crank, no smoke

I've been having some troubles starting my rig lately. It starts, but cranks for much longer than it should. Temps have been about 20 degrees, and I have not had it plugged in. Batteries are brand new within 1 month. Truck cranks over much faster than it ever did prior to the new batteries, so I don't think that is the problem (I forgot to check the cranking rpms. I'll check that next time, but I am sure it is fast enough). I have checked the glow plug relay as per the post by KLH, and it is working. Fuel filter changed within 1000 miles. Stanadyne added at every fuel-up.

From what I have read on here, it seems that since I am getting no smoke on startup after long cranking, then I am not getting fuel to the cylinders. Is that a correct assumption?

Assuming I am not getting enough fuel, what are some of the possible causes?

My best guess is my oil is too thick in the hpop and not letting the injectors work correctly. I am running Delo 15w40 dino. I don't have much over 1000 miles since the last change, so I don't really want to change it now unless that is definitely the problem.

Do you guys think it is the oil weight, or something else? Any other tests I can do without a scanner?