ok mine's a 916 but after returning from a Ducati workshop a while back I had oil running on the oil cooler, burning on the exhaust downpipe and running out of the fairing, I suspected oil cooler lines and it seemed to be mainly around the banjo bolts ('coz they had replaced the hoses) but it turned out that they had cross-threaded one of the rocker cover bolts (on the LH side of front cyl there is an extended bolt which acts as a support for the Radiator) and oil was leaking from the threads, as the shop was a long way from home I looked at it myself and I managed to repair the thread and refit ok and it's fine now.

Now a reputable Ducati workshop shouldn't make that sort of mistake - but they did.

Don't know if 998 has same type of bolt but it would be worth checking them anyway.