Followed till the end
Well I have read every single post on this thread, and thought I would "chip in" with my own little experience.
I used to have a 98 Golf IV TDI 110 cv, that had the 2nd gear crunch ever since it had the gearbox removed for some noise at 24000km. Didnt happen all the time, but i never got it to go well really, a bit notchy in 2nd gear specially in cold (between 0ºC and 15ºC). Ended up using the VW gear oil G052726A2 which helped a lot(mineral I believe), and sold the car at that.
Now I have a brand new 2007 Audi A3 sportback with 2.0 140cv with a 6 speed manual gearbox. Was always notchy in second (identical feeling to the Golf), but just slightly, since it was new, I complained until I got the gearbox replaced at 10.000km. It went really well, and gear shifting was great with the new gearbox. At 40.000km had the dual mass flywheel replaced due to some noise when turning ignition off. After getting the car back with the new DMFW, the notchiness came back, and has been there ever since. Now the car has 49.000km, and the gearshift mechanism has been realigned over 15 times (by me, fine tuned to perfection... and I must say that adjustment does make a difference, but doesnt eliminate the problem). Now It still feels a little notchy in second and a little in third at 12-15ºC (even more in third than in second), still havent tried it on a really cold morning, but i guess it should be worse. Went to the dealer, to the parts guy, gave him my chassis number and he told me that the factory fitted gear oil my car used was the VW G052171A2. Had the clutch replaced 4000km ago to see if it got better (all this in warranty by the way).
A friend that is a mechanic, told me that audi and vw gearboxes have a "floating" primary shaft (translated), that spins freely, as oil is cold, and thicker, the secondary shaft beats the oil around which in turn, moves the floating primary shaft and pushes it to rotate (it should be stationary, hence the crunch when cold). Once it is warm, the oil is less dense, and doesnt offer resistance to motion of either of the shafts...hence primary is stationary and shifts are smooth (does this make sense?)
Now I am waiting to take it to the dealer on friday to have him check the pendulum support and engine & gearbox mount alignment (as adviced by a vw audi Expert I got hold of).
Mentioned the extra fill up through the reverse switch hole to the Audi exper, and he said that it might help in some cases, that it would be worth the try to add another 200ml.
This is my experience. Will keep you updated as from next week when i get the car back.