After further research and checking out how the oil circuit works, I think the simpliest option is actually the first suggested, a T-piece. It might not have a fast responce when the temperature changes, but I think I can live with that.
However, here we go with another issue. The T-pieces I found all have a M10 x 1 thread, which is good because that is the thread used in the oil circuit (for example for the pressure senders). I have a 150°C VDO oil temperature gauge (
http://vw-t3-bus-shop.de/en/vdo-oltempe ... eige-52-mm" onclick="window.open(this.href);return false;), and the only VDO M10 x 1 sensor (VDO 323-801-009-003D) is too long to fit any of the T-pieces I found online. I could make a proper t-piece, but I took some measurements in my engine and I think a bigger t-piece (that allows for the VDO sensor) will not fit.
So looking around I found that back in the days VW used this sensor: 1H0 919 563. This is a low profile, M10 x 1, 150 °C oil temperature sensor. However, I could not confirm it works with the VDO gauge.
So, anybody out there knows if the 1H0 919 563 sensor works with a VDO 150°C oil temp gauge?