Adding a mechanical oil pressure gauge

I have a 2001 Cherokee 4x4 with 4. OL inline 6. I just purchased the vehicle. After driving it for a few days I noticed that the oil pressure seemed to be low. After 30 minutes of highway driving the pressure would drop during idle to about 10-12 psi. I took it back to the dealership where I purchased it. It has a slight warranty so I had them do the work just in case. They put a mechanical gauge on it and said that in fact it is running about 17-20 psi at idle when warm. They replaced the sending unit, but this did not fix the problem. After consulting with a jeep master mechanic it was concluded that the gauge is probably the problem. Obviously to replace that would be costly, and the master mechanic said he would not worry about it. However, it seems that some of these cherokee are susceptible to oil pressure problems, so I am not comfortable with that. I would like to add a mechanical oil pressure gauge full time, but am curious how I get around the check gauge light issue. Any ideas on how to do this. Would a simple T-connector work where I could leave the electronic sending unit connected and also connect a manual gauge, and does anyone know if these are common and easy to come by. I can't believe I am the only person ever to want to put a mechanical gauge in a car that has an electronic one. Any thoughts or suggestions would be appreciated.