Hey...it's been like 25*-40* here in Va for the past few weeks. What's the best oil weight for a tired old 400 in my '78 150 4x4?

 

Here in Mn we get a lot of cold weather. I always run full synthetic since its a better oil and doesn't change the thickness as much when it gets cold. I run 5W-30 in my 75 truck with a 429 but its a newer build with great oil pressure. Never goes below 40 psi. My 92 f150 with a 302 I tried 5w-30 but the oil pressure was lower than I liked. It has 138,000 miles on it. I would recomend running 10W-30 or 10w-40 full synthetic for a 400 with lots of miles. That is what I would run in MN.

 

If its only 25-40 as a low I wouldn't worry about running any different oil than you do in the other parts of the year. It isnt until you hit single digits that things start getting really bad. 7 degrees here this morning, ol girl fired up with full synthetic 10w-30.

 

I ran 20w50 year round in my old cougar with a 302, but it had issues with lightweight oils. Oil pressure with a 5w30 would drop to 25 to 30 pounds, 20w50 was always 50psi. So there is no correct answer its all about your engine and what it needs. If you don't have a proper oil pressure gauge to monitor the changes just stick with a 10w30, and as someone said above synthetics don't get as gummy when cold or as runny when hot.
P.S. Castrol synthetic is on sale at O'Reillys right now.

 

Agreed, that's not that cold, i run Rotella T 15w-40 year around and have started at 40 below zero (with the aid of a block heater).

 

At those temperatures it is not to going to make a lot of difference.
Each engine is different and the more wear your engine has the more need for thicker oil. Multi-viscosity oils handle this very well.
Do not use an oil with low zinc content in a flat tappet engine or you will risk ruining the cam.