Handful of fun issues

[Dark5yde]

1) Lets start off with what I hope to be the easy one. I keep getting the "check engine oil level" on the dash. My oil level is fine. I'm hoping its a bad sensor. However my mind is playing tricks on me hearing noises and what not. Is there something else that could be possibly happening? ie. Oil pump or something?

2) After some severely heavy continuous rain, I notice a water sloshing sound from my rear air vents. When I put A/C on max I even feel a slight spray. Could this be the product of a under hood/cowl vent having a hole or something allowing water in somehow? This has never happened before.

3) P0735 - Gear 5 incorrect ratio, I took a drive (85-90miles each way) pushing 70-80mph almost the entire time. When I exited the thruway that code popped up and put me into limp mode. I reset the code on the fly got to my destination no problem. When I left same story, towards the end of the trip when I exited the thruway code popped up again. Again I reset it on the fly and got home with no issue. I have never had this code pop up in the past or since.

Any help on 1 or all of these issues would be greatly appreciated.