Or about half that, if you have a Harbor Freight nearby.
https://www.harborfreight.com/fuel-pump-and-vacuum-tester-93547.html They sometimes have them on sale for $9.99.
17 seems a bit much, but the important thing is that the engine never knocks (aka detonates, pings, etc) under load. The idle timing is deliberately a bit slow (even without the vacuum retard), for easier starting etc; so the factory advance curve moves quickly at low rpm to bring the timing up to optimum at medium and higher rpm. So if you set for best idle (whether by ear or by vacuum gauge), the timing may be over-advanced at higher rpm; which might lead to knock (or not, depending on a lot of factors like fuel, temperature, engine condition and so on).
This is typical pre-ignition damage, although in this case it was caused by too much oil burning (oil has a really low octane) rather than too much spark advance
So my suggestion would be to stick with the factory recommendation of 10 BTDC (assuming the vacuum retard is disabled or not working).
The oil you are seeing is most likely leaking somewhere else (like maybe the rocker cover gasket) and just collecting around the plug. Unless the tip of the plug is covered in oily black deposits (from burning oil), the oil is not likely to be coming from the cylinder. And even if it was burning oil that bad, you wouldn't be likely to see liquid oil on the outside of the plug unless the cylinder was not running at all.