A mechanic who rode my Panda with me recently told me I should put some "Teflon" oil additive to the gearbox as it would make shifting smoother, since he said he could hear some wear in 2nd gear and that this might help reduce friction. Any experiences on this? What oil additive do you recommend? It's a 141A 4x4 Panda. Thanks

Don't bother. Modern gearbox oil have all the necessary additives needed. Too much Teflon can throw up problems such as making the synchros less effective I've heard.