My brother is a mechanic who studied at NTI. He told me that the reason you don't want to change fluid in high mileage transmissions is because the new fluid deturgent will eat away at the paper clutches that alot of newer model transmissions have. Mine is an 81. Do I have to worry about this. Should I not use new fluid for other reasons? My transmission pan gasket has failed and I'm replacing. Ive captured all the old fluid via extra large tote underneath. Do I use the old or get some new. Also If detergent is my enemy will using a cheaper fluid be better as they probably use less detergent. Also also why did they not have a drain plug?