I just bought a '67 with 12 bolt posi in it. I checked the differential oil this weekend and have a few questions, please:

1. I had to stick my index finger in slightly past my second knuckle to get oil on the tip of my finger. I presume this is too low, correct?

2. What type of oil should I put in? I heard posi rear ends take a certain type.

3. I am planning on ordering a new differential cover at some point. Would it make sense to fully drain and change the oil in the differential ASAP, or can I just top it off for now with the correct oil? It seemed quite dark when I pulled my finger out.

Thanks for the help.

12 Bolt Posi Rear ends have clutches in them. The clutches require a "sticky" oil. Ac Delco sells an additive you can add to normal gear oil . Part number 10-4003. Before you do this, read the label on the oil you are adding. A lot of the new oils have the additive already in the oil.

Sorry, forgot about the fill level. The plug you pull to add oil is also the level the oil should be at. I generally over fill and then let the excess drain back out the fill hole untill it stops and then put my plug back in.

12 Bolt Posi Rear ends have clutches in them. The clutches require a "sticky" oil. Ac Delco sells an additive you can add to normal gear oil . Part number 10-4003. Before you do this, read the label on the oil you are adding. A lot of the new oils have the additive already in the oil.

So, will it say something like "for use with positraction differentials" or something similar, so I'll know if the additive is in there? I presume this may be in a standard bottle of 80/90 weight?

Do know of a particular brand that already has the additive in it so I know what to look for at my local auto store?

Thanks for the feedback.

Any 80W90 gear oil can be used. Most will say that they are compatable with limited slip differentials, but that doesn't mean that they have the additive in them. The GM or Ford additives are the only ones to use. The auto parts store additives just don't work. In your case I would just use some 80W90 to top it off and change it when you change the cover.

the question comes to mind .... why was the rear end low ??? check pinion & axle seals for leaking ....

My '65 has the Eaton posi 12 bolt from factory. In 1975 at about 90,000 miles the posi began giving problems on sharp left turns. I drained and refilled with Sta-Lube 90 wt GL-5. No additive added. GL-5 is OK for limited slip. I guess thats why its GL-5 and not GL-4? Worked fine for me for next 35,000 miles at which time the car went into storage. I bought some Sta-Lube 90 wt GL-4 a couple months back for my roto-tiller, so they're still in business.

My $0.02

Any 80W90 gear oil can be used. Most will say that they are compatable with limited slip differentials, but that doesn't mean that they have the additive in them. The GM or Ford additives are the only ones to use. The auto parts store additives just don't work. In your case I would just use some 80W90 to top it off and change it when you change the cover.

Excellent. Thanks for the reply. I was worried I might damage it if I topped it off with regular 80/90.

I will be changing the rear cover and putting in fresh oil in a couple months.

the question comes to mind .... why was the rear end low ??? check pinion & axle seals for leaking ....

I believe the pinion seal may be leaking slightly. I'm going to pull the drums off the week of 4th of July so I can check the axle seals.

With my luck, I'll probably top it off and it will start leaking worse.

I've never changed a pinion seal. Don't I have to drop the drive line and take that huge nut off the pinion? If I do, I have no idea how to pre-load the nut once I've replaced the seal.

Do a search on this sight for "pinion seal" and you will find a lot of information on changing it.

Do a search on this sight for "pinion seal" and you will find a lot of information on changing it.

Thanks.