Nissan Frontiers are famous for this. The O ring where the oil cooler connects, does exactly what someone else mentioned. The heat cycles harden it and when winter comes it shrinks in the cold and you get drips. It was probably 20 minutes to replace. Once the gasket or o ring is to that point, no high mileage oil or additive promising to fix it... will help at all.
You can either pay a mechanic or crawl yourself under and around until you find it. If you already have oil stains, I'd suggest laying cardboard, like a Uhaul moving box for $2, under the truck. Then, before you start it, get directly under it and trace it up from there. Oil cooler, lines, and other connections are tension fittings. Tension fittings aren't bothered by temperatures. Your leak is from something pliable, like a gasket or o ring. Those seals are compression fittings. I'd guess, oil filter housing.