I don't think you'd worry about the gearbox or the driveline generally, things tend to happen a little more slowly, noises come more gradually and there'll be shiny stuff in the oil... when you change the oil regularly. A head gasket can just blow, inevitably when you least need it to happen, certainly when it's a stinking hot day, and most likely when you're on the highway... miles from home. Guaranteed NOT to happen at home in the garage next to the cupboard where the spares are kept. ;)

