to put the oil in the lawnmower's crankcase would be fine, so long as it is an appropriate viscosity.
if its regular gas
Drain the oil and gas out of the crankcase and put oil back it should be OK
Apparently you didn't crank it up. You are still here. Nothing except it may clean out some old dirty oil. Remove the gas and the oil and discard same. Refill the oil section and refill the gas tank. Don't do that again.
You get engine damage from lack of lubrication
If the lawnmower has two compartments one for gas and the other for the oil, then put gas in the gas and a 30w type oil in the oil compartment. Simple as that. If you have an engine with only one compartment then you'll need to check on the manufacturers suggestions which is usually 40:1 or something close.
SAE 10w30 weight synthetic oil.
Yes it is possible to do that.
liquid. think of what you put into your car.
0.6 litres
You can cut down trees with your lawnmower. Warning: Do not actually try to cut down a tree with your lawnmower. More than likely, the gas in the chainsaw is mixed with 2-cycle oil. If your mower also has a 2-cycle engine, nothing will happen. If your mower has a 4-cycle engine, the engine will probably smoke (bluish in color) until the oil is burnt up during combustion. At worst, the oil in the 2-cycle oil/gas mixture will foul up the spark plug and you will probably have to clean it pretty often until the oil is burnt up.
Automotive oil is identical to lawn mower oil. They are both internal combustion engines. I would however recommend you use 100% Synthetic oil in that air cooled lawnmower engine.
Do you mean "mixed gas" as in oil/gas mixture? Then unless it calls for it then the answer is no. If you do and it is a normal 4 stroke engine then it will cause excessive smoking and will eventually foul the carburetor and combustion chamber up. Good luck.