FWIW, oil doesn't stick on surfaces forever, so an engine that is not run periodically, unless prepped for storage, is at risk for moisture accumulation, potential acidification, and pitting of parts. It also will produce more wear on the next cold start. Each engine manufacturer will have a timeline on what they consider long-term storage. On the i3 REx, the maintenance cycle should satisfy that IF you let it run, and note, it CANNOT run until your SOC is less than 75%. Also, once it starts, it wants to run until the engine reaches full temperature, which takes generally at least 10-minutes depending on ambient conditions.
Pretty much all BMW engines were designed to run on a full synthetic oil. For their ICE vehicles, if you don't hit the condition based service, they'll perform an oil change annually...and, that's probably a good idea even on the REx.
The owner's manual should tell you the recommended oil. Since mine is a BEV, at least in the on-line version, it doesn't include any REx info, but you should be able to check that if you put in your last 7 of the VIN, or look at the hard copy. On their other ICE vehicles, the general recommendation is 5-30w, Castrol, European that meets the LL01. The BMW branded version is BMW TwinPower Turbo 5W-30 Engine Oil - 1 Liter (made by Castrol). Modern engines tend to have tighter tolerances than older ones, and the lighter oil is useful to flow through the smaller clearances at normal pressures.
