I would suggest looking at Redline MTF. These oils are GL4 only. This means they have to pass a yellow metal compatibility test. So they should not damage the synchros or bushings.
synmaxperformancelubricants.com/PDFs/SynMax_Tech_Oil_Weight_%28Viscosity%29_Presentation.pdf
Now look at the chart on page 3. Often people run a 20w50 engine oil in their Rover gear box. Looking at the chart you can see that a 75W 85 gear oil is the same viscosity. Hyundai sell a good GL4 oil 75W85 which I think is as good as the redline. Redline I normally buy from eBay, you can buy the Hyundai oil online, plus Hyundai dealers may sell it to you.
I have not had good experience with Penzoil Synchromesh fluid. I ran it in my Rover before the 5 speed conversion, and I also ran it in an Elantra before I had to change the gearbox. There was an oil leak in the Elantra so this is not the only factor. I think this type of fluid is too thin perhaps for splash lube boxes. Also it is mineral and people tend to never change gear oil so it may break down too much before it gets changed.
I have read that Royal Purple gear fluid eats synchros and has poor shift quality. Also I think it is better to avoid GL4/GL5 oils. I have heard of synchro problems with such oils. GL5 did not supersede GL4. GL5 is a specification relevant for differentials, GL4 is a spec relevant for gearboxes. It is the stinky sulphur compounds that eat your synchros. Interestingly we add copper compounds (think brass yellow metals) to wine at low rates to get rid of stinky sulphur compounds.
James.
