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Most factory bolts are stretched when they are torqued and won't hold torque after the first stretching. Or at least that's my guess dealing with new LS family GM engines. Seems that would also apply to a gearbox as well.
Ray's right. The used bolts no longer meet original specifications. They have been stressed. For the price of new bolts. Buy new. Would you use a condom twice? I wouldn't. Thats why I'm not paying child support [-X
Realistically it's probably OK - I've used old bolts on gearboxes before - but never on something like a flwheel or a clutch. They're spinning fast, containing lots of energy and as Carlos says, if the gearbox falls out you might end up with a child and be paying for it!
They may not stretch,... but it is a possibility. There are two reasons to use new bolts:
1) the original bolt is in a high load situation but is not a "torque to yield" bolt. Normally, these will never give you any problems but it is common place to replace bolts in a very high load application with new parts, just in case.
2) the bolt is a torque to yield bolt. This means that after an initial torque is applied, another torque usually consisting of a degree of movement rather than a measured torque, is applied to the bolt. The elasticity of the bolt then is used to apply an added degree of torque to the given part. Examples of torque to yield bolts are Head bolts, some connecting rod bolts, bell housing bolts, flywheel bolts.
Maybe this is a reason: I asked someone who has had a Golf for a long time. He tells me that the gearboxes of this car used to fail because rivets sheared. Maybe fatigue is a reason and VW is just playing safe.
I googled for "gearboxes rivet Golf" and this is what appeared:
