What was found
A/C 765 FLT 404 EN ROUTE TO FRU OUT OF LEJ, FLT CREW ADVISED THE APU OIL QUANTITY IS DECREASING AND EXPECT THE APU TO SHUT DOWN SHORTLY. WITH THE LEFT GENERATOR DEFERRED INOP AND THE LOOSE OF THE APU GENERATOR THE A/C WILL ONLY ONE ELEC SOURCE, AS SUCH THE A/C WILL RETURN TO LEJ. APU OIL LEAK DUE TO FAULTY O-RING AT OIL FILL CAP, FOUND APU OIL CAP DOWN AND LATCHED. SERVICED APU WITH 4 QTS OF OIL AND REPLACED FAULTY O-RING. RAN APU NO LEAKS NOTED. OPS CHECKS OK.