What oil should i use?

If you live up north, use 5-W30 in the winter time. And the rest of the year 10-w30 will be just fine. As far as normal oil vs. synthetic... I personally dont think its a good idea to run synthetic unless you've used it since the engine was new (but thats an ongoing debate in the automotive world!) And brand wise, well thats a matter of opinion too. I use regular Mobil (not the Mobil 1 synthetic) and ive never had any issues in my high mileage (178k miles) Trans Am.

As far as the oil type causing the check engine light, I cant imagine it causing it to come on unless you had the wrong amount and casued oil pressure to drop too low. may want to head to a local auot parts store and have them scan for codes