well, when you have the old plenum off you should have easy access to the oil level/temperature sender (shown in the third image down in this article):
Pelican Technical Article: Boxster Engine Sensor Replacement - 986 / 987
i'd love to know the resistance to ground of the contact that the green/red wire connects to. if it is around 10 ohms when the engine is cold then the sender follows the VDO 10-180 ohm standard and i should be able to splice into it and connect a standard VDO temperature gauge.
so, if it's easy for you to check when you're doing your plenum then cool, otherwise it's all good anyway; i can get to it eventually. cheers.