Handful of fun issues
 

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.

For:
#2 - Maybe the water drains are clogged. You can use some long steel wire and try to slide it through from top to bottom.
#3 - I would try the easy stuff first (which might be the correct solution): you might want to change the ATF & filter. The code you posted P0735 happens when you have one of these conditions: ATF low level, dirty ATF, shift solenoids issues, other mechanical problems, electronic issues.

If the filter gets clogged (old), you get low level ATF symptoms, which would fall in the: dirty ATF & low level ATF (because you starve the tranny).

Just follow the Bentley for this one, it's not very difficult. Be careful to have the 35-50°C temperature (trick: when the tranny has the same temp as your hand, you are at around 37°C - from this point, it will heat up pretty quick to 50°C - that's when you top off your tranny).

changing the ATF worries me. Not at the mechanical ability of doing it but the not so good results I have read about other people changing ATF in these things.

No worries about changing the filter / fluid. just replace what you took out with something that is comparable. No need for BMW fluid, unless you have more $$$ then common sense!

My e39 would have the jerking syndrome when shifting from drive to reverse or vice-versa. Also, hard jerking when downshifting hard. Another issue was some slippage when climbing low grades like underground parkings.
The issue was cured changing the ATF & the filter. The ATF was black as tar, the magnets were 3-4 times their size due to contamination. The filter was getting probably clogged.
After doing 2x the ATF in a span of 2 weeks or so, the issue went away. This was in 2007. This year I will do another ATF & filter change.

Nothing is "lifetime".

Oh I know that. Just odd as I am not now nor ever had tranny problems. Just that 1 trip.