Routine Maintenance fluid changes 1998 Subaru

I have a Outback wagon. My DD, I have 138000 miles. I have my oil an radiator fluid changed on a regular basis. However, my mechanic will never change my transmission fluid or differential fluid. I have been using him for years. Do I need to Change my transmission fluid or differential on a regular basis. I know the Subaru dealer would.

YES…It should be changed. And it’s well overdue. If your mechanic won’t or can’t do it…then either take it to the dealer for find a good independent shop that specializes in transmissions. Stay away from the chains like AAMCO.

Yes these fluids should be changed on a regular basis.

First consult the Owner’s Manual. Consider what it says as a minimum.
I would change trans and diff fluid every 30,000.
Brake fluid every 3 years.
I would also drain and refill the power steering reservoir once a year (an easy DIY project).
Use only fluids specified by Subaru. Avoid the “universal” generic stuff.

I would advise that if this car has an automatic transmission that extreme care be used when servicing the transaxle. (proper name for the transmission but that’s nitpicking I suppose)
The transaxle uses both automatic transmisison fluid and hypoid oil with the former going into the auto trans and the latter into the final drive. It’s important that these not be confused by putting the 2 different fluids into the wrong compartment in the transaxle.

I would suggest following whatever maintenance schedule the manufacturer recommends. We have one car which recommends a great deal of preventive maintenance (like radiator fluids) every 30K, and also recommends high-mileage items like shocks or timing belts be replaced at certain intervals. It sounds like you have a decent mechanic if you have been with him all these years.
We also keep records for this maintenance, in case the dealer forgets to enter it on their computer. Good luck.