Changing rear diff fluid and have option paralysis

[TheRuley]

Truck is a 2001 ranger 2wd v6. I'm going to do the rear diff fluid mostly because the guy i bought it off of did some towing and some camping...maybe some water crossings there might be milkshake inside i don't know so i'm just going to do it so i know its good.

Factory spec is 80w90, but i want to run a full synthetic as winters here on the canadian prairies can get down to -40 and stay there for two weeks straight.

I use my truck to get to work and back every day, which is a 15 minute drive down a country road. Truck doesn't see too much more action than that...maybe the odd highway trip every now and then.

Here's where i am hung up-there is no 80w90 FS available in my small town, but there is 75w90 OR 75w140. I don't want to overthink this too much, i just want something that is going to be close to factory spec while also having something that will be OK in the winter time.

 

[drtyler]

The 75W-90 will be fine.

 

[Kira]

I was in the same boat ('05 Ranger 4WD V6) but have an LSD (4.10) and was spooked by using a synthetic in a long in-service diff with factory dino. Yes, I thought of the old "synth causes leaks in old dino lubed systems."

I settled on a 75-140 conv. with LS additive because many people said it wasn't something to sweat either way.

 

[dnewton3]

Given the expected cold temps, run the syn 75w-90. Any brand will do.
Pick one that's on the PRI QPL list. (see the sticky in this forum)

 

[Jimmy_Russells]

I'm not even sure 80w90 full syn exists? Maybe one or two brands. The vast majority of full syns are 75w90. They are interchangeable.

 

[Overfill]

I'm not even sure 80w90 full syn exists?

you're correct, it doesn't. The bottle has to list the lowest cold flow winter grade it meets, and when a blender makes a mainstream GL-5 gear oil product with synthetic base stocks to a 90 weight, it will meet 75W. Those blends don't really need viscosity index improvers to be a 75w-90, and without them, it's appreciably shear proof.

 

[hank2]

Not a Ford, but I recall that the '08 Silverado I used to own spec'd 80w90 for differentials. but did specifically call for 75w90 in Canada. Much older Dodges or Rams called for 75w140 but I have no idea what late models use. I believe that 80w90 was the very common weight for a very long time in conventional or blend gear oil. Seems to be all 75w90 or 75w140 in syns.

 

[TheRuley]

Changed it last night. Went with mag1 75w90 FS (staff discount rules)

By the looks of it someone had already done the job somewhere probably not too long ago. Old oil still had a hint of golden brown to it as i was watching it drain into the pan. No debris or glitter or sludge. Gears are good no chips or signs of severe wear.

So i probably didn't actually have to do it, but now I know and that was worth it to me.