I don't recommend any of my experiments until I have proven them out on myself.
So, 150,000 miles after I started using synchromesh gear oil in my 02J, I can safely say the Pennzoil, Valvoline or The GM product work just fine and at 1/2 the price of the touted oils, like Redline Amsoil and so many others; some for as much as 3x the cost. The claim by many is that same product made by Pennzoil is manufactured for at least two other marketers. But if it says Synchromesh, it's the same thing it is NOT synthetic. There are times that is a fancy way to say 'It's more expensive', when it doesn't have to be.
The business with ANY bottle of gear oil that says it's compatible with GL-4 or GL-5 is an absolute LIE. The difference is brass synchros compared to steel synchros. The GL-5 has more phosphorus and Sulphur, which are the elements under heat and excessive pressure are the sacrificial element in the oil that attaches to the synchros, breaking off to protect the surface of the synchro, but the GL-5 with the higher amounts of P and Su attach so tightly to the brass synchros, that in the older transmissions with brass synchros, the brass tears off with the protection... not a good thing. Your gear oil will look like you are panning for gold.
Unless you have the wrong stuff in your tranny, I don't see much advantage is 'flushing it'. You are just flushing money down the drain.
Now a Separate topic....
While I am at it, there is an intriguing issue with the '06 0A4 5-speed transmissions. I am not positive, but this may continue with subsequent transmissions.
Pulling the drain plug will not empty the 0A4 transmission, nor will trying to fill the front plug really fill it. In order to completely drain the 0A4, the rear plug and the diamond-shaped shifter bottom support have to be removed. To refill, you can put fluid into the front fill hole, but it won't completely fill. For some reason, there is a baffle inside the transmission that holds oil at a high enough level that it's about even with the front fill hole. So, to fill, put in as much as you can, then turn the engine on and put it into gear.
CAUTION! There is a danger with running the gearbox and wheels moving while working under the vehicle. ONLY perform this with the vehicle carefully and competently supported and being cautious of the rotating wheels. If you don't feel safe with them moving, remove the wheels.
With the gearbox moving, the gears will draw the new oil onto both sides of the internal baffle. If you don't follow this plan, you will only 1/2 empty and 1/2 fill your transmission. Also, do not reinstall the lower shift support with RTV, or it will leak. I use Hylomar sealant.
