My car ( 4-cylinder Nissan Sentra) was introduced to synthetic oil @ ~ 90,000 miles. My oil light went on @ ~7,300 into using the synthetic oil. Had to add ~2 1/2 quarts of synth blend oil. Why did my car burn this oil? My mechanic had said that " some cars burn synthetic oil when first introduced" is this true?

This link may help answer some of your questions:

Ask Mobil 1 Archive - Switching to Synthetic from Conventional

I've never had a problem introducing cars or trucks to full synthetic engine oil, even at 180,000 miles. In fact, Pennzoil Platinum even solved a tick in a 1994 F-150 that had been there since a friend bought the truck. If the engine is in good mechanical condition, there should be no problem switching to synthetics.