Dr T,
The answer to your question is Yes.... It is very common to see increased oil consumption when a high mileage engine is first switched to synthetic. I'd recommend reducing the filter change interval by 50% for the first 10,000-15,000 miles after changing over. It's not a bad idea to do the first filter change after 2000-3000 miles, as it will tend to load up with sludge. This dirty oil provides a poor piston ring seal, so consumption can go up significantly while this cleaning is taking place. If you have a significant layer of sludge sitting in the oil pan, the oil level will also drop as this is loosened up and filtered out.
Once the pre-existing sludge/varnish/carbon deposits have been removed, oil consumption should be lower than with the same SAE grade of petroleum oil. The reason for this is that full synthetics have much lower evaporation rates than Group I/II petroleum oils.