dex coolant is fine if you keep it full, air in the system makes it sludge up. so swapping to green or yellow is a good idea. but do a full flush of the coolant, then swap over, mixing is not good, cause more sludge.

500 aint to bad for a lim job.

how many miles do you have? sc oil can be found at the dealer, or on line, use only gm sc oil. you need 2, 4 oz bottles.

and check your coupler out for slop, change if needed, then add new oil.