Most likely, RF is being induced into the oil pressure gauge wiring. The wire length may be resonant on the 2m band thus providing a high level RF voltage into the ECM's analog input.
I assume you already checked the oil pressure transducer's connector for corrosion or a bad connection.
Try to put some "snap-on"
RF chokes on the oil transducer wire or the oil transducer wiring harness. It's best to wrap the wire(s) around the choke several times it the wire is long enough. If it's short, simply put several chokes over the wire or the wiring harness. Also, placing a choke as close to the ECM (the engine computer) will make it more effective but unfortunately it's not always possible. Just place it where you can and see if it helps.
Next step would be to try "grounding" or should I should say - bonding the engine to the body and the frame in several places. I like to "ground" the exhaust pipe to the chassis in several places with a flat 1/2" ribbon.
If this does not help, perhaps adding a 10 uF capacitor between the oil pressure analog input and the chassis ground may help to eliminate any stray RF.
Adding a bandpass filter to the radios output may cut off some harmonics that may potentially cause this issue. I would do this after trying all of the above.
Hope this helps some.