http://images.thesamba.com/vw/gallery/pix/1296770.jpg
There is some credibility to a few mechanics' complaints that "GL-5 oil degrades the VW moly synchro rings" .. but from what I can deduce, these were aftermarket (NOT VW brand) moly synchros, and off-brand "GL-5" gear lubes with "cheap" being the car owner's priority. I've certainly never seen a moly coated synchro damaged by Swepco or other high end oils.
As a point of reference, modern Porsche 911s (post '86) have moly coated brass synchro rings of EXACTLY the same makeup as VW, and Porsche has always recommended using GL-5 oils. The difference is that in Porsche world, the owners know to use only recommended GL-5 oils, and to stay away from cheap FLAPS junk.
"VW R&Ps have a unique hypoid that is exempt from the standard wisdom of using a GL-5."
This is simply false information. The 10mm hypoid of the VW Type II R&P is the same 10mm hypoid found in a 360-hp Porsche GT3 race car. The fact that the Porsche R&P is 2-3 times the size makes VW's 10mm hypoid FAR harsher by comparison. The ONLY reason the VW R&P can tolerate GL-4 is the low horsepower WBX engine. Install a 230-hp SVX engine, and actually use that extra horsepower against a GL-4 filled transaxle, and you'll see what I mean.
I won't argue that generally speaking, GL-4 oils shift easier than GL-5 oils. RedLine happens to be the easiest shifting oil of any I've ever tried. But I would caution against making choices based solely on ease of shifting. EP protection should also be a consideration, if using any engine more powerful than the 2.1.
From a VW technical bulletin dated 1973:
“The composition of a transmission oil has a varying effect on the service life of different parts of the transmission. For example, oils which increase the life of synchromesh units have the opposite effect on the ring and pinion. The selection of the correct transmission oil is, therefore, always a compromise between requirements which are often in complete opposition.”
There's an internet "expert" who has for years been pushing his anti-GL-5 campaign displaying selectively chosen "destroyed by GL-5" parts. It's rigged "proof", and I've personally busted him on this. He avoids any further debate on the subject, and won't even tell me exactly which transmission type his samples come from.
Paul Guard