Fons,
From the Pilot's Notes: (FMk6, in this case, so this also applies to FMk3 & T5. Don't have access to the little-engined Lightning notes)
"Air to air refuelling sorties of up to 11 hours duration are permitted.....
......attention must be paid to the rate of consumption of other aircraft fluids, e.g. oil and oxygen, based on consumption rates recorded from preceding sorties."
MB
Entirely as an aside, most folks I've spoken to who have flown both the clean F-4 (training fit) AND the F Mk6 (war fit!) against each other in 1v1 ACT found them pretty similar in terms of fighting endurance.