Take a look at the rear top of the engine, the side closest to the driver. There are two cams, the intake cam is covered by the rotor and cap where the plug wires are attached. The exhaust cam has the round rpm sensor or cam position sensor.

The most common leak will be found at the exhaust cam, the oil will dribble out and then follow the lip around the top of the engine and then drip down the side closest to the firewall. The exhaust cam gasket or rubber donut is easily replaced.

The reason behind the cam gasket leaking is usually the result of increased crankcase pressure. Meaning that the PCV system is clogged somewhere, and needs to be serviced. If the car is a turbo, I suspect the the PTC nipple is clogged. For NA engines, it is usually the PCV valve or the small vacuum line attached to it is clogged.

The P0301 code is a misfire in cyl #1, closest to the passenger side. The P0300 is multiple misfires detected. Neither should be related to the oil leak. I would first check the spark plugs and make sure they are seated properly and clean with a proper gap. Along with the plug wires being secure on each plug and the cap.

Klaus
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Volvos should be saved from the junk yards, not donated.