I just had my gb rebuilt, and have the option of adding either mineral or synthetic oil to it. What are the pro' s and cons of each? i have been told
that as its new its fine to add synthetic, and it will lower friciton, has better heat range and also will allow a better shifting feel? these all
seem good, are there any negatives?
cheers
Some synthetics can cause rubber seals to shrink allowing leakage. Check with your GB builder whether the seals used are compatible with synthetics. Apart from that the only down side is additional cost.
yeah it was my gb builder who asked and recomended it, so jsut checking what other peoples opionions are
its 25 a litre to get synthetic 
I added synthetic in Andy's GB after the re-build (2L kombi) for those reasons.
I never had any problems with the previous box on mineral oil (also re-con and did ~60,000km on it), and can't say I've noticed any difference on
this one with synthetic oil. Done ~35,000km on this box now and no issues.
So to answer your question, to date I can't say I have any problems with either. To say it shifts better when cold, I think you have to jump from one
car to the next side by side to notice any difference. I have not tested anything to see if it's lower friction or anything.
I suppose the cost is not that different because synthetic will last longer between changes, doesn't break down as quick etc.
As they are both re-con GB's and no idea how many km's either has done in it's previous life, and both are still in good condition I have no way of
knowing if either oil will extend the life of the GB.
Maybe for your use (if your racing it?) synthetic will handle heat and hard use better than mineral. For normal every day use I think get which ever
oil you take a fancy to.
:blah :blah :blah
If You can afford the Synthetic oil....
Go for it.....
I suppose We all should be going for it...
My Son's will not use anything else in their motor bikes...
Engine & gearbox....
Lower friction is the main issue.... and VWs have a high friction Differential,
thats why Hypoid oil is normally used....
If You read about how good [?] mineral oil is .....
Everyone would be using Synthetic oils....
IMHO..
Lee
http://community.webshots.com/user/vw68autobug
hypoid is used in diffs because of the extreme pressure created by the wiping action of the drive angle of the gear set if you want to lower the amount of loss in the gb and diff ----stick a tube of molybond in ,this is the greatest additive you can ever put in a gb or diff and at 10 bucks cheap as shit