Hey fella! hope i can help. firstly and fore-mostly, it does not sound like you are running low on gear-box oil. If this was an issue
related to the quantity of gear-box oil, your gear-box would most likely be equal to scrap by now. the SMG hydraulic fluid level
could be low due to a leak in the system but again, that would manifest itself in shifting problems in all gears most of the time.
you could still check it as a precaution anyways. so basically, the only realistic possibilities you are left with are a faulty SMG pump
unit, a faulty SMG control unit, a faulty SMG shift unit, faulty SMG gear position sensors, or an internal/mechanical fault in the
gear-box.