Figures for a Lycoming IO-360-A3B6 for the two series before and after changing to 15W50. 8 qts added at oil changes, thereafter oil usually added on reaching the 6 qts level, or a bit before if the next flight's a long one. Note that W80 bottles were 1 litre whereas 15W50 were 1 quart. Also, the profiles of flights vary quite a bit with a mixture of short local hops but also quite a few long airways trips:
Aeroshell W80 oil change at 480.0
1 litre added after 9.9 hrs, 14.0 hrs, 3.0 hrs
Aeroshell W80 oil change at 512.8
1 litre added after 8.0 hrs, 8.1 hrs, 8.0 hrs, 11.5 hrs, 5.2 hrs, 5.5 hrs
Aeroshell 15W50 oil change at 567.7
1 quart added after 20.6 hrs, 9.0 hrs, 3.6 hrs
Aeroshell 15W50 oil change at 607.0
1 quart added after 16.7 hrs, 10.1 hrs, 4.8 hrs, 6.6 hrs, 6.4 hrs
Those are all the figures I've got. Further oil change due now.
Looking at the figures now, there may be a litre missing in the first set - 14 hrs looks remarkably long.