Leaking valve seal

Pull the spark plugs, if there is that much oil being burned, it'll show on the plugs unless they are newish.
It's unusual for any quantity like you mention to leak past a guide while the motor is at rest as the oil would have to climb up the guide to run down inside the guide, that'd have to be a very loose guide, and the head oil drains would have to be plugged to accumulate oil, which I have seen.
If you find a oily plug, there will likely be "ash" build up one one side, that side will face the bad guide, and yes, exhaust valves can suck oil with venturi vacuum .
We're it mine, - I'd replace the guide seals, tighten the intake manifold bolts, and cross my fingers .
Good luck.

P.S. don't be discouraged if it smokes for a while after seal replacement, the whole inside of the exhaust system will be coated in tar, and need to burn off.