Rear gear lube

Black GT · Aug 17, 2019

I have a slow leak at my pinion seal. I was in the store today and thought I would buy a bottle of gear lube to top off my rear end. I noticed there is all kinds of different weights. What do I need for my 2001 GT? Also noticed some said they were for limited slip. Does this mean no additive is required for a positraction? Thanks in advance.

stormsedge · Aug 18, 2019

I have 75-140 Royal Purple in mine, but 75-90 would work as well...the Royal Purple "doesn't require" additive, but I had some so put it in anyway. Read the manufacturer write up to decide...pick something spec'd for high performance and you'll be in good shape...I sold Valvoline for years with no complaints. Cheers!

tsemmett · Aug 19, 2019

I want to say I put Lucas 75-90 in mine when I had the same issue (pinion seal leaking). I didn't worry about the additive (I didn't add that much), but I also intend to rebuild mine in the near future (incorrect pinion bearing preload has caused the gears to howl). If you don't, you might drain it and give it a clean fill with the proper additive (regardless of what the oil says).

Black GT · Aug 19, 2019

I bought the 75-140 royal purple and topped it off yesterday. Like to have never found the fill plug. When I found it the muffler was in the way so I had to use some plastic tubing on the tip and squeeze it in slowly. I have owned the car a year and in that time it was not even a half pint low. tsemmett, I have the same problem with the gear howling. Mine only does it after I shift into 5 gear. Once in awhile in 4th gear. When I bought my car I drove it 150 miles home, all interstate. The howling was so loud I stuck my finger in my ears to get a little relief from the noise.

tsemmett · Aug 19, 2019

Mine howls if I let off the throttle. From what I can tell, this means the pinion bushing preload isn't right. There's a crush sleeve on the pinion that sets this, and you have to torque it down to a specific point to crush it correctly; mine was over-torqued (confirmed when I replaced the pinion seal). My father did change the gears in it long before I got it, but he must not have rebuilt it fully (30+ year diesel mechanic, so I'm not sure what his deal was). I'm eventually going to gut it and upgrade to 31 spline axles/differential and 3.73 gears (has 3.27s now).

Shuttleman83 · Aug 19, 2019

Change fluids correctly first with basic fluid, I'm not sure I believe in additives to be honest. Tried to power steering additive stuff and it really screwed things up for awhile.

tsemmett · Aug 19, 2019

Shuttleman83 said:
Change fluids correctly first with basic fluid, I'm not sure I believe in additives to be honest. Tried to power steering additive stuff and it really screwed things up for awhile.

Not quite the same thing in this case; limited slip differentials require the additive, and call for it from the factory. You run the risk of damaging the differential by leaving it out.

Shuttleman83 · Aug 19, 2019

I mean fluids that advertise extra additives, not basic fluid which requires it.

Northwest_stang · Aug 25, 2019

Shuttleman83 said:
I mean fluids that advertise extra additives, not basic fluid which requires it.

Power steering additives are made for power steering fluid. Aren’t all sn95’s on trans fluid for the power steering? I know mine is. Might be why it got screwy for a min.

Anyways my thoughts on additives is basically cheap insurance. Sure it may not need it but for less than 20 bucks I figure why not?

Black GT · Aug 25, 2019

The whine in my cars rear end is almost not noticeable since I topped it off with lube. Was only a half pint low but I guess filling it up did the trick. Before adding the half pint I avoided interstate driving because the whine almost hurt my ears. When I bought my car and drove it the 140 miles home I occasionally had to put my finger in my ear to get a little relief from the high pitched whine. Happy it has lightened up now.

Northwest_stang · Aug 25, 2019

So id keep checking it if you keep it full then do a full fluid change in 500-1k miles you might stand a chance for the whine to completely go away. If not then it’s just out of tolerance and ideally should be changed.

Look at it this way it’s a good excuse to change the gear ratio for a better, funner car

Black GT · Aug 25, 2019

Northwest_stang said:
Look at it this way it’s a good excuse to change the gear ratio for a better, funner car

3.73 here I come. lol

Northwest_stang · Aug 25, 2019

That’s what I’m saying lol and there’s always a few people that say forget 373 go 410 or you’ll be sorry.... to me it’s just not worth it. Because 410 your almost completely killing 1st gear usability maybe even 2nd, the high rpms on the freeway(I think it’s 3k @75mph?), and the added gas cost as such but I also drive a crap ton of miles cuz my average is about 30k a year so all about weighing your pros and cons because there is a noticeable difference in the gear pull from 373 to 410

tsemmett · Aug 27, 2019

3k rpms should put you around 80, but it's definitely higher than I like. My truck's overall gear ratio is really close (.79 overdrive with 3.55s) and I hate the highway RPMs.

Joe Jr. · Sep 12, 2019

so i learned from a guy doing a gear swap for me in the past that the additive that ford recommends was added to make the gear oil "slicker". So he used synthetic oil and didnt need to add the modifier because the synthetic is so much slicker than dinosaur oil. So, i'm guessing if you don't add the modifier or use synthetic the clutch paks wear faster.

tsemmett · Sep 12, 2019

Whether or not synthetic oil is good enough on its own seems like a topic that has been argued to death, but I see it this way: Motorcraft Friction Modifier is roughly $15. A new clutch pack is more like $75 (not including new gear oil and the time/labor to replace it). I'd play it safe and use the additive, synthetic or not.

That said, many synthetic oils already contain these additives, and that could be what he was talking about.

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