Re: 15/40 or 5/40 synthetic?
I switched to Rotella 5-40 synthetic for the winter. IMHO
...it made a huge difference in cold starts. It doesn't sound like it coming apart. My block heater is bad so I haven't plugged it in yet. (I have a new one, just haven't gotten to it yet) I live is southwest VA, it has only been to the 20's a few days here this year.
97 F 350 CCLB DRW, 3in dp to 5 inch Diamond Eye, zf5, 4wd conversion, intercooled, 315/75/16's
E99 F 350 CCLB 4X4, Diamond Eye 5 inch exhaust, boost, pyro, oil temp, zf6, 6637, PHP Hydra, lifted on 35's, Valair clutch
09 Harley Ultra, Rush pipes, Stage 1 tune