I am just curies why would you bother to change a fluid in your tranny at all? Factory manual recommended to check oil level first time after 100,000 miles. Just check not to change. My truck has 169,000. Have had no problem yet. Should I do something? Or just keep out of trouble?
This is the prevailing mindset of most people today which is completely ridiculous. You can get lucky and not have any issues as you have done, I agree that happens, but for me the gambling of that is not worth it. ALL fluids degrade over time and transmission oils are no different, especially for automatics which are even more critical to keep changed IMO. I've seen a lot of people lose automatics and I ask "did you ever do ATF changes?" and they say "no, why would i need to do that?" and that is all I need to hear to explain the situation.
I change all my fluids quite regularly, including my diff oil, PS fluid, brake fluid, coolant, etc. I think brake fluid is quite critical because it is so hygroscopic and picks up water so bad rusting brake components and lines really bad.
On your truck you could probably change it and it won't hurt a thing since it's a manual, but if you had an auto i'd say keep gambling at this point. Bear in mind that changing the fluid also helps get any metal shavings and bits out with it (wipe the magnet plug clean when you change). Typically on high mileage automatics after no fluid changes new fluid just goes in and wreaks havic by breaking dirt and stuff free clogging valve bodies, etc. If kept up with regular changes to begin with the transmission would stay clean and it's the best scenario. But on your manual it'd probably be fine.
