Cold(ish) Weather Oil Viscosity 318 Poly

[64D100Poly]

Hey, starting to get a little cold here in the PNW, It averages anywhere between 50 to 25 maybe warmer or colder. My truck is my daily and every morning I start it it'll stall from the oil pressure and I have to restart it, no big deal but it's in need of a change and I want to run something thinner anyways, right now I have high zinc Valvoline ZR1 or whatever its called in 20W-50. What should I run? It's got a decently built 318 Poly 320ish hp solid lifters still decently low compression and only has about 15k since rebuild. I did buy a PCV setup to run the 2 breathers it has on it right now are causing to much water vapor and sludge to form in the valve covers and crankcase so hopefully that will help that out. Any recommendations are greatly appreciated, the original manual doesn't really mention an oil that covers the temp range, and even then oil tech is completely different now from 1964.

[PwrWgnDrvr]

10-40 or 5-30.
I only ever ran straight 30W in my poly/LA's here with many a night in low to mid 20's.

[dodgeboykim]

I run 15/40 in my rebuilt Poly and i live north of you near Sumas, Washington. Temp here today is Plus 2 C/35 F.
I also run 15/40 in my 97 Ram 5.9 Magnum year round.

[martincom]

Mobil 1 Full Synthetic 0-40w would be a good year around choice. The zinc level in it is more than adequate. As it is a full synthetic, it has a full synthetic price. As long as your truck doesn't have an excess oil consumption issue, the price is offset by the extended change interval, 10K miles.

[jeffc57]

been using 15/40 diesel for years 20+ in both 70-la318 and my 66-a318, use it in all my old mopars