I am going to put some royal purple gear oil in my 150's axles this weekend.What weight should I use.I have a Dana 44 in the front and a Ford 9" in the back.

 

I don't believe you have a choice with RP on gear oil weights just use their synthetic gear oil it's the right stuff.

 

most all of the newer ford rear ends have a metal tag on them that says 75w140 Syn oil only. i'm sure royal purple makes 75w140. i know valvoline and mobil 1 do

 

The factory fill on the 9" must've been 80w90, so 75W140 should be good. The front needs a bit lighter oil, though. Here's what a TSB says:
# TSB #BC0131960102 -- 4X4 FRONT AXLES MUST USE HYPOID 4X4 GEAR OIL P/N F1TZ-19580-A & HIGH TEMP 4X4 FRONT AXLE & WHEEL BEARING GREASE WHEN SERVICING. IT IS NOT RECOMMENDED TO USE HEAVIER WEIGHT LUBES IN THESE FRONT AXLES. (NHTSA ID #46438, DECEMBER 01 1995).

The Ford description is a bit (?!?) vague about the weight -- I think 75W90 is the closest match.

 

Why would you use 75W140 in the rear and something lighter in the front?

 

I do not know. Maybe the tolerances are smaller in the front axle.

The actual reason for a different oil is not stated anywhere, as far as I know. The service manual simply states the F1TZ-19580-A gear oil for F150/250 front ends, without ever specifying anything more, and that's it. (For the F350 front and most other rears the oil is 80w90, and that's obvious from the Ford part number.)

From the scant info available on the F1TZ-19580-A oil, it appears to be a straight 80 weight mineral oil with 10% EP additives. Using a regular 80W90 or 75W140 is unlikely to make a huge difference, but I know I'll be using 75w90 next summer when I regear my front.

 

The rear is usually carrying a bigger load and is often limited slip, so lubrication requirements can be different. Most towing, for instance, is in 2-H, so all the load is going through the rear axle. Also remember that the front is made by a different company, Dana.

Jim

 

Dana makes all the front differentials for Ford?

 

No, the newer IFS setups use the Ford/Serling 8.8. All TTB and probably all solid axle setups were Dana. Some 1960's Ford 4x4's used Dana 44's both front and rear. I re-read the original post, and he has a Ford 9", not 8.8 in the rear. I dont think 140 was ever specified for the stock 9", but I could be wrong. It seems that 75W-90 should be perfect for both.

Jim