I have a feeling you may have oil and temp sending units made for idiot light, not for analog gauges. The idiot light sending units have a simple Go/No Go sending unit circuit, whereas their analog gauge related counterparts will report a "degree of" measurement change throughout their entire range.
Getting an infrared thermometer is a great first step, but I have found it I am more than 8-12" away from the object I am checking temp on, the reading will begin to get lower as the thermometer InfraRed sensor is impacted by relatively cool air between the the thermometer and the target object.
Another possibility is the sending unit wire(s) got pinched and grounded. It is also possible your Instrument Voltage Regulator (IVR) has failed. The IVR is attached to the back of the Instrument Panel, and in nearly all cases it required the instrument panel removed.
While the panel is off you may be tamped to replace the old incandescent illumination bulbs with LED lights (which are far brighter). That is a good move, but there are a few things you need to be aware of. First, unlike incandescent bulb, LED lights are polarity sensitive, so if one (or more) do not work when installed, just remove the offending lights, rotate their position into their panel holes by 180 degree, and try them again. Also, LEDs are far brighter than their incandescent bulb, so you may find the LED lights to be a bit brighter that you want. I have read that the conventional dimmer (rotating the headlight switch shaft clockwise or counterclockwise) does not work with LEDs, but their may be circuitry options to allow the LEDs to be dimmed. I have never needed to do this, but I keep that tucked into the back of my mind/
Attached are some PDF files that may be of help to you.