You guys have me wondering about my use of 15W40 year-round in my 84 SD. It got to 8 deg F about a week ago, I had forgotten to plug in my block heater, and my car really didn't want to turn over (took about 4 tries of cranking). It seldom gets below zero around here.
I'm considering using Shell Rotella T 15W40. My independent uses Castrol 15W40, but I'd prefer something with an CG rating at least. Another mechanic once told me that it's ok to mix oil brands, even viscosities; basically saying it doesn't matter.
Even if this is true I'd prefer to keep use same brand of oil. I understand Shell makes Rotella in a 10W40 but I haven't seen it for sale anywhere, and I've never seen a mail order source for oil. Sorry if this has been covered before.
[This message has been edited by Robert W. Roe (edited 01-22-2000).]