I go to the drive in quick lube place, where they have a "vacuum" that sucks a differential or trans dry and let them change it - you can look at what they get out to see how gritty it is .... OTOH, I've had 50 year old trucks that I'm sure have never been changed and they're still working

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics