Could a guy T the probe into the oil feed line on the turbo?
I have my oil temp probe T'd into my oil return line from my bypass oil filter. The probe is a good seven feet away from the engine by the time the oil flows from the turbo oil supply feed down on the bottom of the block up across my engine and through my bypass filter.
After a half hour drive my gauge reads about 170, it will go up to 180 if I'm sitting idling or doing a lot of town driving.
I did have a problem with mine when I first installed it, I bought a traditional 3/8 inch T for my routing of my fittings for other purposes of just getting the oil back where it needed to go without needing adapters for that. For the part of the "T" that the oil temp sender was going to go into, I had to by a 3/8 to 1/8 bushing as my probe was 1/8 ntp. By the time I threaded everything together, the probe wasn't actually getting a good enough "flow" over the surface to move the needle. I tried turning the T upside down so the oil would have to flow over it more since the flow out of my bypass system really isn't "pressurized" much, but didn't really help.
Instead of using the bushing adapter and keeping the probe from seeing as much oil flow over its surface, I bought a 3/8 coupling piece and drilled and tapped 1/8 threads into it so the probe stuck the whole way into it and the oil would flow better over it.
I guess my point with all of this, is the place you mounted it (as I'm not 100% sure where you mean you placed it near the oil filter) might not have a good enough flow to raise the temperature of the gauge. Could just be the gauge as well.