I finally took my newly acquired LS for full fluid service, motor oil, transmission, diff. However, my mechanic told me that the drain bolt on the diff was rusted on. Not sure how bad but he said he was able to get the fill bolt loose and the fluid looked ok. What would you guys recommend? Force the rusted bolt out? Or is it possible to suction and just refill from the fill hole? Is it ok to mix oils? Never really had this issue before.

 

Hit it with some penetrating oil regularly for a few days.
After a few days apply some heat it should come out…

 

Not sure how mechanical you are if you're paying someone to change fluids, so the answer depends a bit. For sure try the PB blaster SOAK for a few rounds and a small torch to get it to pop (use a breaker bar w extender).

If not, its not the worst thing to just leave diff lubee alone. But I would try to break it loose and then order a new bolt/crushwasher if you get it loose.
Also usually a mechanic shop would be able to break it loose for you (possibly weld a bigger nut and get a better bite).

 

Make sure to clean out the hole in it for the hex key and when you go to remove it, insert the hex key and tap it with a hammer. Hitting it will drive it further in and also help shock the rust loose. It should be a 10mm hex, definitely recommend using an Allen socket with a breaker bar as well.