Oil also cools engine. But I don't know how long the oil alone can keep
the temp in safe range.

True, but some area's like around exhaust valve seats become very hot
very rapidly without aggressive coolant circulation. Oil circulation
alone will provide little if any cooling around these 'hotspots'.

Even full of coolant without circulation, these area's will quickly boil.

Personally, were it mine, I'd limit dry runtime to 20 seconds or less,
and then only under light loads. If I 'had' to move it further, I'd give
it 10 or 15 minutes between runs.

Keep in mind that even during normal operation, said hotspots are
'pushing' localized thermal shock limits.

http://en.wikipedia.org/wiki/Thermal_shock

Erik