rear differential service schedule

Recently had my oil changed and the guys were telling me that rear diff fluid should be changed.
Any thoughts on this? I realize it is a thicker viscosity but does it need changing at an oil change or maby just once a year?

Any ideas and if so what's a good brand of oil ?

I've had my B2200 18 years. I try to change it about every 30K miles.

I drain and then fill with Valvoline 80W-90 gear oil, and use the pump that screws into the top of the gear oil container. You should have a 23mm socket for this, and always loosen the fill plug (on any vehicle) before you remove the drain plug.

I use the same gear oil in my manual transmission as well.