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.).
Mental illness can occur due to defects or trauma to the brain as an adult or during fetal development. Long term substance abuse may cause anxiety, depression, or paranoia. Examples of other biological factors that can trigger mental illness are poor nutrition and exposure to toxins.