Why would oil pressure go from 40 to 80?

Answer 1

Oil tends to "lighten out" when a motor comes up to full operating temperature and will often show a drop - for example fro 80 to 40 psi - when slowed from road speed to idling speed such as at a traffic light. As engine speed picks up when pulling away fro the light, the oil pressure should rise again. That's normal.

If the oil pressure fluctuates like that at road speed, there's probably a problem in the oiling system. Fortunately, the most common source of the problem is also the cheapest to fix because the problem is often caused by a problematic oil pressure sending unit rather than the lubrication system itself. If oil pressure fluctuates wildly and it's not the sending unit (or the gauge itself) the candidates for the cause get much more exensive, such as the oil pump. Since these problems are often within the motor's internal working parts, they can get rather expensive to repair. But they're still far less costly than a new motor, which ignoring problematic oil pressures can lead to.