is your oil pressure sender equipped with an extra pin for the oil pressure switch? Mine has one so I just removed the original oil pressure switch and fitted the sender there.
Now the original pressure switch AND the new oil pressure gauge are BOTH connected to the new sender.


If you don't have a sender with an extra pin to connect the original switch, you could also buy a t-piece for that!

It's a standard thread (i don't remember the correct size but I'm sure it's an NPT thread - I will have a look on the workshop manual)
A mate bought his t-piece from a big industry-machine company, they said these are standard for hydraulic lines on big machines.

So I guess it won't be that hard to get one...