How often do I have to change synthetic tranny fluid for 5-speed?

 

Probaly after 75k miles or never. The Redline product will last much longer than the petroleum product. After 50k miles, take a sample and send to Blackstone Labs for an analysis and pay the $10 extra for the TBN (Total Baseline Number) to see how much longer that it will last.

 

For normal driving, synthetic gear lube can be a lifetime fill. You can change it when you do the clutch and essentially not worry about it.

If you're worried about it, the analysis mentioned above would be a good idea.