I have a very slow oil leak manifesting itself around the extreme upper front left (passenger side) of my engine bay and gradually working it's way down to the bottom of my grill and onto the bottom front bumper. I've had the car on the hoist and no obvious signs of a leak anywhere.

The only possible thing I can think of is something to do with the A/C compressor.

It's also slowly spreading along the left underside edge of my bonnet as it must be getting blown back by the air at speed.

Can anybody offer a clue or had a similar problem?

 

There is oil in the refrigerant used in the AC system. Usually oil on the outside if a hose or line is a leak indicator. You will have to take it to a certified AC tech for repair. AC work is something that can be dangerous and easily screwed up.