Changing to GL4 oil worked for me too , in a 156 20 valve. It's not perfect, but much improved. It took a while to improve though; perhaps it takes a while for the friction reducing agents in GL5 oils to be rubbed off the synchroes(?)
Edit: Just thought I'd add: when I drained the original oil, at around 70,000 miles it looked new. Old gearbox oil always used to come out yellow in my experience, from synchro wear. I suppose no wear is good in a way, but it does seem to confirm that the synchroes aren't being given a chance to work properly with the GL5 oil.
Last edited by R.T.; 22-02-16 at 16:19.