I installed a rebuilt transmission in my 81SD 18 months and 45k miles ago. Put Mobil 1 in there. Hasn't leaked a drop since and the fluid is clear and very clean.
The rebuilder notes the fluid should be changed every 35k.
Question is can I run this any longer without causing wear? Or should I be making plans to change the transmission fluid and filter now? Anybody have information on extending between changes with synthetic trans fluid?