what is the temp range for 15w40?
 

I like to use rotella t 15w40, but was wondering when is it too cold for 15w40? whats the lowest time you can safely run 15w40? I run it in my 87 300 inline six and have started it as low as mid 20s. And i was also wondering about running it in a 72 model 302 v-8 motor. Would 15-40 be too thick for say the ocasionally really cold morning(mid 20's)?? but most of the time say 40's or higher? Basically how cold can it get and still run 15-40???

later!! chris

Why make you engine work much harder at cold start up with a 15w40 oil, in 40-30-20 degree temperatures? Your starter, and especially oil pump trying to pump up that oiltaffy, cold?
Why not put a 1/2 quart of 15w40 oil in your refrigerator for 24 hours. Then pour it out, and actually want 5 quarts of this oil in your engine, during the winter? Take a good look at that 15-40 when it pours...cold...
Why would you even want to run a 15w40 oil, in a 302 V8, Chris? It's a tired, piston-slapping, beater engine, burning blue smoke out the tailpipe, correct?
You like getting poor fuel mileage as compared to getting the peak efficient gas mileage, right? Some guys simply enjoy spending more for gasoline, and stopping more often at the gas pumps, than needed. It's an odd phenomeneon, but I appears some think like you do, Chris. We see it all the time here.
Good luck. Continue to use the 15w40 oil in your 302 V8, a good Ford Motor, by the way. Your engine will love it.

It is quite clear from SAE J-300 that 15W is intended for use down to -20C (+4F). I used 15W-40 regularly in a 1994 5.0 in winter temperatures here (+20F and above) with no signs of slow cranking, piston slapping, lifter tapping or any other oil-related problem.

The only oil that miserably failed a "freezer test" was Coastal API SA non-detergent SAE 30. The streaks of wax were visible in the gel. Delo still poured, but I would not use it at 0F, since it is not rated for that.

Jim