If you're on PC you'll find that this issue has pretty much gone since patch #2, if you're a sensible driver.
Getting hold of the game at 1st, I used to get some smoke trailing my gearbox after a couple hot laps in career, even blew an engine once. All despite my best efforts not to overrev upon braking.
Since patch #2 I can be harsher on the gear box, without any consequences!
(If you're not on PC, wait up for patch #2, it should help with your problem.