I've been trying to track down an oil leak for a while and it seems to be coming from the oil filler tube in the front of the engine and possibly from the timing cover. I have a 76 Olds 455 in a 37 Olds. I was looking at an older thread that had pictures of the engine and noticed that there was a breather on the opposite valve cover. Mine has the pcv valve on the passenger side but the driver's side valve cover has no breather. Both valve covers appear to be stock. Could it be that blowby pressure is building up and causing oil to leak from the filler tube?

 

The pcv should be hooked up to a vacuum so there would be no pressure. I would think you would need a vent on the other side to allow filtered air in. is the oil coming out the top of the filler tube or lower down? I'd decrease the engine so you can see where the leak starts.

 

It's probably blowby. I would replace my PVC and on the drivers side I would install a breather with a tube to the bottom of the air cleaner for some additional neg pressure.

Read this:
https://classicoldsmobile.com/forums...-oil-leak.html