Dillmann, who was a race-winner during a part-season with Rapax last year, finished the quicker afternoon session 0.2 seconds faster than Racing Engineering's returnee Fabio Leimer.
Felipe Nasr, now at Carlin, completed the top three, followed by ART's James Calado and Lazarus driver Rene Binder.
Nasr's Carlin team-mate Jolyon Palmer had been fastest in the morning, ahead of champion squad DAMS' new lead driver Marcus Ericsson and Binder.
Formula Renault 3.5 title-winner Robin Frijns took ninth in the afternoon on his GP2 debut with Trident.
Only 24 cars took part in the test, with iSport absent after announcing last week that it is set to sell its GP2 operation.
Red flags came courtesy of a spin for Dillmann's Hilmer partner Pal Varhaug, electrical problems on GP3 champion Mitch Evans's Arden car, Daniel de Jong stopping on track, and Sergio Canamasas putting his Caterham entry in the tyre wall.