the only thing sealing it is a thread sealer on the threads. Other than that, its most likely leaking internally. There are two halves inside, one side where the oil pressure is and the other side where there shouldn't be oil. There is a diaphram in the middle and when it starts to leak, oil will seep out. I believe its the same switch as the ones on the 2nd gens, and if so, its a 1 1/16" deep socket to get it off. Its not a hard fix so don't sweat.
I'm not that great with metric conversions, but your best bet is to go to the parts store and take the new switch and fit it to a socket. If you don't already have that size, pick it up when you get the switch. I did some looking and see different part numbers for yours and mine. But the pics are identical.
Good to know. I've got a lamintated conversion chart on the shelf above my tool box at home and it comes in handy. There's only a couple I know of off the top of my head because I've had to go back and forth with them: 3/8" and 10MM are very close and 9/16" and 13MM are also close. Beyond that, I gotta look them up. Yeah they are only like $10 or $20 and it just unscrews from the block (unless you have an oil cooler it might be slightly different). One thing I will point out is to make sure you clean the inside of the connector/plug that goes on it. If it is leaking like they do, the oil is all over the inside of the electrical connector and if you don't clean it good, the oil light will never come on when its supposed to. I had to clean mine twice before I got a good connection. To test it, all you have to do is turn on the key but don't start it. If the oil light comes on like it should on a non running engine, you should be good to go. If it doesn't, clean it again. good luck.