From the 912 Line Maintenance Manual
Section 12-20-00 Page 48
Draining the suction lines, oil cooler and return line is not necessary and
must be avoided, as it results in air entering the oil system. Therefore
see and carry out SI-912-018. See also chap. 12-20-00 section 11.6)
Section 12-20-00 Page 49
11.2) Oil Change
The engine must not be cranked when the oil system
is open. When the crankshaft was turned, then the oil
system must be purged.
12-20-00 section Page 56
Purging the oil system is necessary:
- before each first start up (e.g. after overhaul)
- after maintenance work during which the lubrication system was
opened and voided.
- when engine was cranked with open oil lines (e.g. during oil change). (my bold)
- when oil system or oil lines were blown through with compressed air.
SI-912-010 Oil Change
CAUTION: Do not turn the engine if any part of the oil system is open. Observe this also during first installation of the engine (e.g. when mounting the propeller, after the oil system has already been correctly vented).
I think it is safe to assume that an emprty (drained) oil tank counts as an open system which could allow air into the oil system.