I bought an 05 R53 a couple months back and one of the upgrades done was an aluminum radiator. Previous owner says the temp used to be around 190-200F and after the addition it ran in the 170s and 80s during summer while cruising. Cool, no issues there.

My question is, is it normal for the coolant to be running around 166F during winter or is that too low?

I recently changed my oil with Mobile 1 0w-40 for winter and since then my Service Engine Soon light has been on and throwing a P0128 code that I cannot clear with my scangauge 2. To my understanding it only throws this code when the engine doesn't warm up within a certain time. Ever since it has thrown the code the weather has been up and down (40F to almost 70F) but there has been no real change in engine temp.

Coolant level is fine in the reservoir and the oil temp doesn't seem to have changed much but all I have to go by is the stock gauge cluster. I'm tempted to try changing back to 5w-30 but I don't want to waste the money if that wont help.

 

On the face of it, I'd say you have a low-rated thermostat, or perhaps none at all.
It seems that the early MINI t'stat is a 195F one.
I have an '02 R53 & I've been watching coolant temps in the last couple of weeks using the 'secret menu' in the OBD system. It seems to stabilise at about 96C / 204F after about 4 or 5 miles. The gauge shows Normal in less than a mile & stays there!
It's cool in the NE so no opportunity to get it really hot.
If you're in a warmer area you may want to stick with what you have, otherwise put a new thermostat in.

 

It needs a new thermostat! They all die, sooner or later and at some point it will get stuck closed, then you will have wished you replaced it!