CALGARY—One of the best-performing oil companies in the past year is gaining favor with investors in part by embracing an unusual strategy: promising not to reinvest in its core business “in the foreseeable future.”

The company is Suncor Energy Inc., Canada’s largest oil producer, and the core business is the country’s controversial oil sands. A host of big international oil companies have plotted a speedy retreat from the region because of concerns about how the once-booming area can remain lucrative in the face of regulation...