The oil temperature gauge is pretty much the engine temperature gauge for the 3 series.
If the oil temperature rises to 75% or higher, then u have a problem. Normally it should be around 55 to 60% on the 3 series.
If you are so paranoid about having to know the engine temperature, you can always install an aftermarket guage. But, it would pretty much be useless, as the built-in oil temperature gauge is good enough.