I have a Sienna 2000 XLE with 168000 miles on it. I bought it new and it still runs absolutely fine.
Her transmission and differential were drained and refilled at 90000 miles service by Toyota dealership in 2005. The pan was also dropped and cleaned.
Since then, I started changing transmission fluid myself every 15000 miles / year by sucking out the fluid from dipstick using a 12v vacuum pump and adding back the new fluid in the same amount that came out. I usually get 2.5 qt out this way. Since I don't get complete fluid out this way, I do this every year to keep the fluid close to fresh if not absolutely fresh. The differential doesn't get drained (or sucked) in this process.
My questions are
1. Since transmission and differential share the feed, does differential fluid also get refreshed by means of circulation/diffusion as the car is being driven ? OR it keeps the fluid in an isolated cavity after filling it the first time ? I may have to drain differential if later is the case.
2. Is this way of changing the fluid OK ? I am doing it for last 4-5 years and the van seems to be running fine until now at least.
3. Is drooping the pan necessary ?
Thanks in advance for your responses.