As odd as it sounds, one thing I'd look at is the end of this sensor wire in the e-box. It heads into one of the multi-connectors in there.
I found oil under my connector and my mechanic said he'd seen it before where the sensor leaks into the wire, pushing oil up inside the insulation...that might affect the voltage it's showing.
Also, nothing personal, but there's also a temp sensor on the ofh...I have no idea if they're interchangeable, but it wouldn't surprise me if they were...so you sure what you took out looked the same as what you put in? You sure you reconnected whatever you disconnected?
Also, since you're having the dealer change your oil, I'd mention that I think your car might get to be very expensive to maintain...oil is very easy to change and it gives you a chance to look around underneath yourself...which is always a good idea!
Among my other random thoughts, I'd say you should check your pulleys and belts--and since your idle is low (s/b around 700) I'd read up on cleaning the icv/tb and making sure you're otherwise up on 'Inspection' work--plugs, filters, etc.
Sounds like because you're idling so low, your oil pump isn't doing the work the car wants it to do. I doubt the oil is wrong, but it couldn't hurt to ask...
Oh, finally, don't think anyone mentioned this but you should check your oil level, of course! Next time you change the oil yourself, btw, might want to calibrate your dipstick in case it's the wrong one. If your oil level is just at max level on level ground in the a.m., and everything else is fine, then you might consider trying to add just 1/4 qt. of oil.
The red oil light can be oil pressure, but it can also be just a low oil level.
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Mangofy your E46!