Hi, this is my first post to USMB. My current project is an 83 Brat, 131k miles, sat for the last 10-15 years in Texas, probably in the direct sun.

The oil pressure gauge shows nominal pressure when the engine is cold. Over the first 10 minutes of running, the oil pressure eventually drops to zero as the temperature comes up. Here's what I have tried:
- drain the oil looking for sludge; fill with oil and sludge remover and run 10 minutes; fill again with new filter
- switch to 20W-50

Didn't find any sludge. The heavier oil helps a little (I think), but still the pressure goes all the way down to zero.

I'm convinced that the oil pump is working, else I should see zero pressure when the engine is cold. I could replace the oil pressure sending unit, but why would the sending unit malfunction only when warm?

Has anyone else had this problem?

Idle oil pressure will be very low.  No load on the bearings anyway,  so no problem.  Stay with 10w 40.  Don't lug it, these engines are made to do higher rpms.   The factory guage is not accurately calibrated,  especially for the pressures at idle.

Thanks @DaveT. I guess I should clarify that I'm driving the vehicle up to 60 MPH while warming up. When all warmed up, I can rev to 5000 RPM and the oil pressure needle moves a little, but settles right back at zero.

40 minutes ago, DaveT said:

The factory guage is not accurately calibrated,  especially for the pressures at idle.

Are you referring to the gauge inside the dash? I have an 87 Brat for parts, maybe I'll swap in that gauge. It was working properly 4 months ago.

The stock gauges are only roughly accurate while driving, if they are working.

Idle, they will look like zero.

For real checking, get a mechanical gauge.

If it's reading very low while driving on the highway, it could be the sender or the gauge.  Or some kind of real problem....