I need to find an oil leak

Clean all of the leaking oil off of the engine. If you have a steam cleaner that would work the best, but you could use a pressure washer or a couple cans of carb cleaner.
Pour some tracer dye (available at most auto parts stores NAPA for sure) into the crankcase (where you put the oil).
Start your truck and run it for a few min.
Park it in a dark place and use a black light (also available at parts stores or online) to find the fluorescent yellow tracer dye. It will be coming out wherever there is a leak.
This is the best way to find those pesky hard-to-find leaks.