Manual: In a manual transmission, the problem is not so much the fluid degradation, but rather fluid contamination. This contamination occurs over time as the synchronizers, bearings and gears in the transmission wear out. The resulting metal particles then float around in the lubricant.
A lot of mental illnesses can have a genetic component, yes (eg. depression, bipolar disorder, anxiety, antisocial personality disorder, etc.).
Sometimes mental health disorders have biological causes. Some of these biological risk factors include chromosomal and chemical abnormalities, genetic predisposition, medical illness, poor response to medication, poor sleep, substance abuse, brain damage, and already having a mental health disorder.