My Nissan dealership just changed my oil and switch my car from a 5w30 to Mobil 1 synthetic oil. I have 90,000 miles on the car 2011. Will this be good or bad for my engine ?

I think what your asking is the dealer removed your 5-W-30 Mobil 1 synthetic and installed their 0 or 5-W-20 bulk oil! If this was one of those discount oil change specials you should not expect a high dollar oil to be included during the service" Mobil 1 can exceed $7.00 a quart and Amsoil Synthetic can exceed $14.00 a quart so when you can have a oil cost of $80.00 to $100.00 without the filter you shouldn't expect top tier synthetic oils to be included with a $36.95 oil change special!

Call JD at the Magic Mechanic, I'm sure he can answer that question.