1986 Ford Ranger OIL LEAK

1986 Ford Ranger 4 cyl Two Wheel Drive 269000 miles

Oil is coming from somewhere and I cannot locate it the engine has recently been rebuilt prior to my owning. Oill leaks on driver side it appears to be spraying from somewhere valve cover is not leaking rear main seal is not leaking appears to be in vacinity of oil filter or base of the dipstick does not leak unless at highway speed. I got under and looked wiped away all prior oil and that is the only place I could determine how do I find it and how can I fix it? Thank you