Q: Oil drain bolt leaking

Bought the vehicle 2 weeks ago and decided to change the oil for good measure. I bought a volvo filter and Castrol Edge. I'm not certain I did the filter 100% right. The housing is on tight, but the filter may be free spinning inside? Just seemed strange the way it went in. But the bigger issue is this: The drain plug was rounded off but not leaking. Rounded bolt remover tool got the oil drained. Went to Autozone in spare car. They sold me a B18-1.5 I think. It was a bit short on length and the head size is now 3/4in. I install the washer and screw it in the oil pan. Didn't seem to have problems. Add the new oil, start it up. Drips right from the bottom of the drain plug; faster as the car gets to temp. I tighten the bolt up more, as tight as I dare go (fear of breaking bolts - have done that in the past. A lot) That seemed to slow it down, but I still need an empty oil quart to catch the drops. In 4 days it's leaked 1/4 of a quart so it isn't gushing. What do I do?

My car has 100000 miles.