Been using T6 5W40 and NAPA Gold 1515 filters for years. Change out every 6 months (Jan. and June). Noticed shortly after Jan 2021 the oil pressure gauge took longer than usual to register which is normally immediately. Around April 2021 I replaced the filter only (1515). Still had a delay. Fast forward to early this week. With no T6 to be had I switched to Travelers (Tractor Supply) 10W40 conventional oil and an FL1A filter. Five quarts of oil and one quart MMO. Oil pressure gauge was back to normal. Just started it today after sitting for two days and again gauge acted normal. Temp here is -3 with the wind chill. I saw a recent YouTube video that showed a NAPA filter using a material other than silicone for the anti-drainback valve. I have no idea if that was the issue with my gauge or not but I figure I should post my experience.

 

10w40 and -3 doesn't sound like a great idea to start with

 

Ambient temp was in the mid teens. With highest wind gusts factored in the temp was -3. Unusual for around here anymore. 40yrs or so ago it was a lot different.

Per cruiser54:
Jeep Engine Oil Viscosity

This was taken from pages 82 and 83 of the 1989 Chrysler/Jeep owners manual.

Selecting Engine Oil Viscosity

When changing or adding engine oil select the proper grade by using the chart on the following page. Select the grade that corresponds to the ambient temperature range you expect to encounter before your next oil change.

30*F to 100*F+ 20W-40 or 20W-50
0*F to 100*F+ 10W-30 or 10W-40
-20*F to 60*F 5W-30