I am going to swap out my 2.73 gear for a 3.73 richmond gear. I have posi and was wanting to know what everone is using. I have used original GM with limited-slip additive and had good luck, but I am thinking of using a synthetic or blend. Maybe royal purple or castrol GTX gear oil. Also with a synthetic/synthetic blend do I still need to use the limited slip additive? What is everone else using and having good luck with? thanks

GM Differential Lubricant and posi additive. That's what the dealer gave me. Doesnt have the weight on it though

mobil 80 90 synthetic with 2.5 bottles of additive

2.5 bottles of additive?

M1 in some amsoil in my others and gm posi additive in all the posi rears

Yeah, I couldn't get it to stop chattering after a day at the strip. So I added a bottle. Went back again, more chatter, added a half bottle. Went back again, no chatter.

There is a place by where I live that deals rearend parts only. They started by adding one bottle. Drove it around after..for about a couple weeks then I noticed some chatter. Went back to the place and they added another bottle of posi additive and the chatter went away. So yea, sometimes you might need 2 bottles.

amsoil 80w90 in mine

brought back from the dead.lol.

Just wanted to get somemore opinions.

Amsoil 80\90wt with Eaton Carbon Plate Posi. No additives...no chatter.

Note: What type of Posi do you have? Auburn's are different from Eaton Plate style. You'll probably need the additive with an Auburn.

I have the stock posi for a 89 RS. It does require the additive. I think I will use Mobile 1 with GM additive.

I use the Castrol Hypoy C gear oil w/ GM LSD additive.

First note, never break in a new rear with anything but conventional oil in it. It will void the gear warranty if you use synthetic.

I've got Amsoil in my WRX in the trans/front rear and the rear. I've got conventional in everything else. After a few more hundred miles I'll make the switch to Amsoil in my Camaro. The rear is still very green in it, so I want to make certain that it has worn in first. I've got conventional in everything else. Due to constant creek crossings I change the gear oil in my Jeep about once a year (7 quarts total), so I stick with conventional to keep it cheap.

So going with synthetic gear lube with a new gear set is not a good idea? The posi unit is the stock unit, it is already broke in. But will it be bad for the gears?

Royal Purple

I just got off the phone with Richmond Gear tech support, they said it would be fine to use synthetic gear oil with a new gear swap!

I don't care what they said over the phone. I still wouldn't do it. That phone call won't mean squat when and if the gears fail from using synthetic oil. Always better safe than sorry.

why would the gears fail from synthetic and not conventional oil? doesn't make much sense to me, it's not like breaking in a flat cam with dino oil.

My understanding is that the gears require friction to break them in properly, which allows heat cylcling the gear metal. Synthetic oil is too slick to allow this process to happen, meaning that the gears will never achieve proper heat cycling during the break-in process. When you hammer on the car the it would be like taking a freshly rebuilt rear and flogging it without ever breaking it in. The gear metal will have a shorter working life, and lower fatigue strength, eventually resulting in a ruined gearset.

It's the same principle as trying to break in a fresh motor with synthetic in the crankcase. The rings will never seat properly, and the cam (if using flat tappets) won't be able to establish a wear pattern.

still not sure i belive it. never seen it written or recommended anywhere by anyone, except you and here.

valvoline synthetic 80w90 with one bottle of GM additive

Some of that information I read somewhere, but the place escapes me right now. Some of it I've surmised from the break in instructions that always accompany gears. They always instruct you to drive a short distance and then let the rear cool for an extended period. The gear manufacturer determined the amount of heat needed for proper heat cylcling and determined that distance using conventional oil. Synthetics will make the gears run cooler (less friction), so in the same amount of mileage the gears won't reach the proper temperature for adequate heat cylcing. If the gears aren't heat cycled properly they will fail prematurely compared to a properly cycled set. This probably doesn't mean in the first 1,000 miles, but down the road like after 50,000 or something, depending on use. I'd just hate to rebuild something and then have it live a less than possible life.

They also usually call for a fairly quick oil change interval, like after 500 miles, so given the cost of synthetic gear oil vs conventional it doesn't make good economic sense to me to fill the rear with costly oil, just to drain and change it shortly down the road.

I can't say with any certainty whether or not that claim is valid. It might be. I do know that most gear manufacturers use synthetic for the first fill in many gearboxes, but most of those are not hypoid gears. Manufacturers of worm, helical bevel, spur, and planetary boxes all use synthetic in their HD units, and worm gears have an even higher demand on proper lubrication and correct break-in than hypoid gears. I also know that ZF uses synthetic in many of their highway use gearboxes and axles, although most of their HD axles are helical and not hypoid for greater efficiency and durability.

The bearings certainly don't need nor benefit from any kind of friction, so only the gears are in question. I know I've replaced lots of side and spider gears and filled with synthetic without a problem. I've repositioned the pinion depth (which would imply a "new" contact area and pattern) and filled with synthetic with no adverse effects.

If your gears are hobbed, shaped, heat treated, ground, and lapped like any GOOD gear is, there should be no benefit to "heat cycling" them once they are installed. If they get hot enough to alter the crystalline structure of the metal, they were either manufactured cheaply (no annealing process) or you have a very serious problem between your rear wheels. If you have inferior gears, or if you have trouble getting the contact area adjusted properly, then there might be an advantage to allowing them to lap themselves together after assembly. I would simply shop for better hardware. If you are interested, I have some surplus gear cutting equipment for sale - seriously. Lieber, Pfauter, and a few odd-ball machines, too.

Since I mentioned the efficiency advantage of helical gearing over hypoid gears, it would seem that anyone using a Dana or Ford 9" rear axle would really want to use synthetic to help compensate for the 3% loss in power transmission efficiency due to the poorer design contact center angles as compared to most ofhter automotive axles. Unless you have 10+ extra horsepower to spare, you're losing power with those axles, and should probably switch back to a 8.75" 12-bolt for some more free power. I know that was off-topic a bit, but since everyone seems to be so excited about using Ford 9" axles, I thought I'd explain why they are slower.

Sorry. I apparently neglected to respond to the original question. I use Mobil synthetic - just like most of the aforementioned gear case manufacturers, like Morse, Boston Gear, Sumitomo/Beier, SEW, Nord Gear, Fairbanks, ZF, Rexnord, Falk, and Cleveland Gear, to name a few.

I agree that the break-in period isn't going to affect the structure of the metal, I don't think that's the reason for it. I think it has to do with the surface heat treatment. Think about it this way. If you take a piece of metal that has just been surface heat treatmed and then heat it up and sustain that heat for a long period of time and then let it cool slowly in an oil bath, what's going to happen to the temper? It's going to leave the metal, and you'll be back to a soft, temper free surface. The break-in allows the surface to heat cylce so that it becomes stable, and resistant to loosing it's temper. If this wasn't a concern why would gears need any kind of break-in at all?

Hypoid gears a very different from any other kind of gear. Straight cut gears, helical cut gears, and other gears don't have the same requirements as hypoid gears in terms of lubrication. Also, rear gears are made from different alloys than other types of gears, so the requirements differ even further.

The power loss through a Dana or Ford rear is really only theoretical. In a back to back test the same car was fitted with a 12-bolt, a 9", and a Dana 60. All rears had the same gears and differentials. The car trapped the same speed & ET throughout the test, regardless of the rear. That basically blows the mechanical advantage of the 12-bolt out of the water. I was further suprised that the Dana 60 didn't lower the times or speed, because it is considerably heavier than either a 9" or 12-bolt. I look at it this way: anything's better than a 7.5" 10-bolt!

Sorry..I don't agree with the theory of (edit)" Don't use Synthetic Gear Oil with new gears...it will destroy the gears and void the warranty ". Bad advice IMHO.

As mentioned, Hypoid gears are already run in and lapped before they leave the factory...in fact, running a Hypoid set in with regular oil to break them in could end up costing you a Ring and Pinion set. ( Edit: Only under extreme circumstances though...see article. )

5 Minutes on the Internet came up with this very good article from " Randy's Ring and Pinion " ( well known to Circle Track racers ) on how to break in a new Gearset.

R&P break-in article

Notice that the use of Synthetic fluid is encouraged during break-in ......(edit) in some circumstances.

More than enough heat and gear loading is produced during the break-in period, regardless of oil used. However, under heavy break in loads, such as in trucks or motorhomes ( or with numerically high ratios ), diff oil temps can get so high that the mineral fluids will actually break down. Then you can kiss your diff goodbye. Synthetic oils will not break down at extreme temps as do mineral oils.

Not the same process at all. See the article from Randy's Ring and Pinion.

I use royal purple and 1 bottle of gm posi additive

I wouldn't use synthetic gear oil for break in why spend $30 on gear oil when you are gonna have to change it in 500 miles I'd just use regular gl-5 for break in then switch to some synthetic


I don't think it would really matter if it synthetic or not because your supposed to go easy on it so it shouldn't see extreme temps or shock(ie high rpm shifts or launches)

you'll never get a correctly installed gear set hot enough to change anything. i doubt you'd even get it hot enough to disapate any entrapped hydrogen and that occurs at a very low temp.

well, you guys have made up my mind..

I will go with Mobile 1 synthetic and the GM additive.

thanks.

Clarifying my position on this...I agree with Kairles. I break my diif in with regular GL-5, because of exactly the reasons he stated. Edit: exceptions as noted in article...IE: High numerical ratios....these will produce very high temps...high enough to break mineral oils down under some circumstances. Also, you can get a proper pattern with some gearsets ( GM for instance ) with very little backlash. This WILL produce a lot of heat.

Some " Professionals " even prefer the minimal backlash theory because it places the loadings closer to the " Root" of the teeth, where the gear is the strongest. Makes sense...but also makes a TON of heat. GM gears can be set-up this way and run quiet.

What I do not agree with is TKOPerformances' statement of:

> Quote: First note, never break in a new rear with anything but conventional oil in it. It will void the gear warranty if you use synthetic

That is the issue I was responding to. If anything, you will be safer with Synthetic. The only downside is additional cost. But what's an additional $30 compared to peace of mind? Hope that clears things up.

Redline Synthetic.
And I broke it in with the same.
No problems, no noises.
-Rich-

I use AMSOIL Series 2000 75W90. No need for posi additive.

About this "break in": Gear lube has a sulfur-based extreme pressure (EP) additive in it. The way it works goes something like this: As heat is generated between the teeth, a little of the sulfur is oxidized and deposited on the teeth surfaces. This sulfur/oxygen compond produces a good slippery surface, which reduces heat production (and wear), so oxidation of the EP additive stops. As the sulfur/oxygen compound wears away, more heat is generated, more of the EP additive is oxidized, a new layer is formed, and so on.

Using petroleum-based gear lube for the initial running will produce more heat in a shorter amount of time, which will also deplete the EP additive quicker. Using synthetic from the beginning only means less of the EP additive is used up.

Interesting article. I'm still not going to break in new gears with synthetic fluid. Even if what is said is 100% true the cost factor is still an issue for me. I'm not spending good money for synthetic fluid just to drain it in 500 miles.

The sulphur/oxygen complex is an interesting tidbit. I wonder if some amount of sulphuric acid isn't produced too though. I mean, the oil is hydrocarbon based, so there's plenty of short chain hydrocarbons held together by easily broken bonds. How difficult can it be for some measure of H2SO4 to be produced? That's probably another reason to use synthetic in the long run, because the types of hydrocarbon chains are very limited in synthetic oils vs conventionals.

Heat generated from too little backlash shouldn't be an issue in a properly set up rear. Obviously less backlash produces more heat, but if you stick to the recommendations for the type of activity your rear is going to see then this shouldn't be a problem.