As far as oil, if you take it to a actual nissan dealer, most dealers dont use full synthetic unless you ask for it exactly. most dealers use a synthetic blend (part synthetic part regular) as stated before if its changed regularly full synthetic really doesnt make much of a difference, the problem with using full synthetic is if you use it once, it effects the internal gaskets differently and its said to be harmful switching back to a non-full synthetic oil after that, and full synthetic every time gets expensive (30ish for synthetic blend and 70ish for full synthetic)

As for transmission slippage, in my last truck (a 1990 ford f150) I had some slippage issues, I changed the fluid/filter and added a special additive from a company called BG (the additive I believe is called BG Auto trans fluid) Im not normally one to advocate all the flushes and additives and all, but that stuff seriously worked, The slipping was gone and all. If you ask for it the place changing your oil should have something similar. hope this helps!

Also, I was a oil change/entry level tech for about 5 years. so i spent my fair share of time around it all