912ULS min Run-up temp issue

Maybe Altitude has something to do with it? I preheat the oil below 50f so not sure how long it takes to get 120 from 30. The other day it was 7 below f I preheated to 47 and it took about 5 minutes to hit 120

Wow. I forgot to mention that I use a Tanis Heater prior to start and my Oil Temp was 51 degrees F. It then took th 25 minutes to get above 100 degrees. My airport, KLMO, is a little above 5,000 MSL

I have never had any problems getting my engine to run-up temp (50ºC) within about ten minutes, even when it's nearly freezing outside; I'm at an altitude of under 500ft, but I'm not sure why altitude should make a difference.

What does your oil pressure look like when you take it to 4000 rpm at only 100ºF?

170ºF seems quite low for cruise or pattern work.  I think you would want to get it up to at least 200 to avoid moisture problems.

Are you sure that your sensor is working properly?

The problem with high revving at low temperatures is that the Oil Pressure spikes.
To speed up the warm-up process, set the brakes and advance the throttle to ~60psi of Oil Pressure.
As the engine warms up the pressure will drop and you can again advance the throttle to hold 60 psi.
By the time you reach 50°C (~125°F) you will find you are Wide Open and still under 60 psi.
Go Fly!

Thanks for your advise