I think you will find oil is going from the vacuum pump to the intake of the engine. You can duplicate the condition by jacking up or elevating the front end sometimes. My feeling is that as the engine is tilted more oil is able to get through a leaking diaphram on the pump somehow.

Disconnection of the return or exhaust line from the vacuum pump may prove this. Also if you think about it most the oil put into the engine from that source would show up in the first couple of cylinders as you experienced. Simply put the diaphram in the vacuum pump is leaking is the prime suspect.

The clue to me is a long time ago there was a member with a simular issue.It took awhile to nail it down. At the same time I of course cannot be positive it is the issue. Just that there is a good chance it might be. If this is indicated to be the problem a rebuild kit for your older pump is not that expensive.