Hi,

I second the thought about the oil pressure switch as it only takes the tiniest crack or just a loosening of the crimp holding the dome of the switch to create a leak from some direct pressures of pump flow.

A valve cover gasket has seepages downward into the spark plug wells and maybe gets farther down the iron block. These have with lots of dirt and gunk buildup.
Clean oil is a clue that it is more than a seepage.
The rear plug mentioned can even manage to leak oil down the back side of the block, run around the oil pan lip, to make you think it’s a pan gasket or even a rear Main seal.

A front cam seal seeps onto the water pumps housing.
If and when the front crankshaft seal leaks the wiring harness, under the engine there, gets very nasty.
If you find that kind of stuff please inspect the timing belt as oil can be very detrimental to them.

If your rubber mounts got soft it’s either the switch or some very careless oil filter changing.
Fresh hot detergent oil is designed to do just that, dissolve and suspend.
I believe synthetic rubbers are made black from carbon pigments.

Was the belly pan holding any appreciable amounts of that engine oil?
This socket and wrench catching invention, or a gravel shield named by from some vendors, is a good place to keep in your sites on with numerous observations.
Oil filter spillage runs pretty much gets to go everywhere and will soak the cross member along with the plastic pan lip.

Getting things back to fresh metal is a mandatory first step.
The worse offender will have the thinner and cleanest oil residue trails and thicker dark ones that resist removal.
The oldest are layered with dirt that’s a convenient bonding or building agent.

I’m sure glad you are pawing all over this car as the payback of some tremendous smiling is ahead!

Phil