For reasons I will never understand, posi additive in 9-bolts is a controversial issue on this site. I am not trying to start a battle here, I am only trying to describe my research and experiences on this topic. It may help some of you make some decisions about your 9-bolt. Replacing parts can get expensive and the method here is a poor-man’s approach, albeit probably a short-term fix. Since it is very likely that my 3rd gen will be sold within the next week and I will likely not be visiting this site as often as I have been (3 times a day), I figured I’d post this for the benefit of others. Do as you wish with this information.

According to the bottles of posi additives that I have looked at on store shelves, the additive is to be used in rear ends with clutch-type posi units. 9-bolts are cone-type units. From a sales/marketing perspective, I find it hard to believe that the producers of the additive would miss the opportunity to mention cone-types if it were really to be used.

According to my research (if you do a search on this forum, you will find the lengthy words from GM from the Camaro Owner’s manual the TSB’s on the topic), posi additive is not to be added to 9-bolt posi units. This is documented in a series of TSB’s (where GM kept changing their mind) with the final TSB stating that this should not be done.

According to the parts manager at a local Chevy dealer (and a guy who autocrosses 4th gens), the only effect that you should see from the posi additive is for the noise level to be reduced. This is not my experience.

According to my experience, adding posi additive to a 9-bolt will weaken the posi “action” of the rear end. The first time that I changed the rear end gear oil on my 9-bolt, I did not know about the posi additive “controversy”. I simply put the additive in the rear. The posi action became less and less pronounced over time (a couple of thousand miles at most and it was gone). Was my posi unit wearing out? Probably. However, I personally believe that the posi additive caused an already weak posi to get worse. Read on.

As a test, I put Red Line 75W90NS in my rear end. According to Red Line’s literature, this oil is “recommended for manual transmissions and non-limited-slip transaxles that recommend 90 WT oils. Can be used in racing limited-slip units to increase lockup and reduce wheel spin. Street-driven rear-wheel drive cars should use regular Red Line 75W90 or 80W140.” Well, my car is street driven and autocrossed. So, I called Red Line about this before putting it in my rear. What they said was that the lack of friction modifiers (e.g. the contents of posi additive) would cause the posi action to be more pronounced, but would accelerate wear on the cones. I decided since my posi was already weak and I have a spare 9-bolt in the garage, I had nothing to lose and I’d learn something. I put this oil in my rear. …and I did learn something.

Just as I was hoping, my posi “came back”. In fact, the posi action was so strong that when going around turns at slow speeds, the outer wheel had a tendency to skip. It took a while for the posi action to come back (a couple of 100 miles), but it did come back. I figured, “hey, liquid locker”. I was pleased, to be honest, though the rear end got a bit noisier.

Now that I am selling the car, I know that the slow-speed skipping would be undesirable to a potential buyer, I added the posi additive the night before last. On slow speed turns, the outer wheel no longer skips and the rear is quieter than before I put the additive in there.

The only real unknown in all of this is whether or not my posi was already worn or not. As it turns out, it doesn’t really matter. I can say with confidence that the posi action is weak with the additive and strong (probably too strong) without the additive. The change in behavior is somewhat progressive when eliminating the additive, but the behavior does change.

One man’s experience. “Your results may vary.”