the chemistry and of a lead acid battery dictates a voltage of around 12V. unless someone has added 4(?) more plates, 18V is not an option. when the engine is running, the generator charges the battery at about 14V, but the battery holds 12V when you stop the engine. its therefore perfectly normal to see a rise when you start the engine, but there is no way that the battery would hold 18V whether the engine is running or not so your voltmeter must set wrong or broken.
as for oil around the dipstick, either its a seal or possibly the level is high or piston rings worn such that air is being pumped into the oil tank causing a pressure rise and leakage. what happens when you take the dipstick out with the engine running? oil "mist" might spray out, but significant amounts shouldn't. good luck