I drained my rear differential fluid tonight and it was quite nasty and really stinky (why does the oil stink so much)? I was contemplating re-filling it with some cheap Supertech gear oil, run it for a few days, then drain it and fill it up with the good stuff. I'm going to be using Royal Purple so I don't want to waste that. What do you think? I do have limited slip. Would I be ok not using any friction modifier with just regular dino oil for about 100 miles? Or will it mess things up? I have friction modifier but it's a waste of $10 (plus another trip to the dealer) to pour it in just for the rinse phase if I can get by ok without it.

 

Try it! It will get grabby if it wants it. Highway miles are the worst, drive it locally and do a lot of turns.
Some gear lubes have the friction modifier already in it. The modifiers make it slicker.

 

You shouldn't need the modifier with the RP. Most high end gearlubes have it already!
Like j-fox says, try it. You'll know (hear it and feel it) if you need it.
I'd use it, if it were me. Good chance you won't need it after the changeout. If you do....$10.00 well spent!
BUT, I also think 10-20 miles would do the "rinse" you are looking for. JMO
Stink is usually from the activated sulfur in the EP (extreme pressure) additive. Smells like rotten eggs..... no?

RJ

 

Limited slip additive is a additive that helps the clutches in the lsd slip easier making the lsd less productive when you need it, but it keeps grandma happy when she is driving. I never run it - I want my lsd to have its maximum potential.

 

Thanks for the replies. I'll go ahead without it for now. I'll just make a few trips around the block then drain it again.

Won't that ruin your LSD?

 

No, it just makes the clutches grabby. I have 240,000 on my lsd and it still works like it should.

 

first answer i have heard that i agree with. was told that in the 60s by a factory rep. some of them will chatter no matter what one puts in them. sometimes i use it sometime i dont. havent had to replace clutches in any of my units yet.

 

It might not wear out anything in the differential running without modifier but it for sure wears out your tires faster.
I ran without modifier for a little while and noticed at the pavement where my driveway joins a circular skid mark that appeared to be coming from the inside of my turns tire. Guess it was skipping trying to keep up with the speed of the outside one.

 

So, if you just want to run the cheap oil for a few days to clean it out, why not just pull the cover off and spray everything out with brake clean? Lift the tires so you can rotate the differential, or roll the truck a couple of feet so the cleaner gets in everywhere and flushes out the crud.

Let it completely dry before you put the cover back on and refill with good oil.

 

Well, I thought about brake cleaner but I don't like putting solvent in there. I guess it doesn't really matter. Anyway, I went ahead and added the Royal Purple and used red Permatex gasket maker (high temp stuff). No leaks and it seems to be working fine. I also didn't add any extra friction modifier to the Royal Purple. It's kinda hard to hear the differential over the straight pipe but I couldn't hear any chatter on tight turns. I torqued the bolts down to about 25 ft-lbs. Haynes manual said 35 ft-lbs. but that seemed to high.