Mechanical gauge issue

Just put in an Autometer Sport Comp mechanical oil pressure gauge. It seemed to work fine for one or two laps around the neighborhood but now it's stuck at 40 psi. even with the motor off. The only thing i've done different since installing it was change the oil to synthetic 10w-30 and put some pipe thread sealer on the tee I made at the sender unit. I was careful to use very little and not allow it to get near the opening of the brass fittings. I have since cleaned that all off and used teflon tape. Could the change in oil viscosity have somehow made this gauge act goofy? The gauge does work as it jumps to around 50 psi when the motor is running.

'70 Fastback Mustang 351w C4

Sounds like the gauge has gone defective to me..I would return it and get another one.

I just sent one back, ditto - sticking at 40 (the car usually ran at 55-60 with the old SW gauge), cut it off and let it sit, it would slowly drop back to 0, then do it all over again.

I was worried as I had dropped the extension on the dirt going out to the garage, and ran Q-tips through it etc to be sure nothing got in it.

Popped in a new one, back up into the 50's and working fine.

Now you convinced me, there is a bad batch of gauges our there, or unless you mail ordered it, and got the one I sent back! Just get another, and have fun. I would be more worried if it had stuck a 0.