Hi GoldwingGuy,
you can find a large set of data in the 'Question of the Day' forum in this site, reading the posts made by Molakule.
I think you will find out that most of the substances added in motor oils sometimes play more than one role in the scene of lubrication, for example ZDDP or moly...
..and weight is a factor, in general thicker oils require more energy from the engine to be pumped and circulated so more energy is consumed for this purpose, which is translated into elevated gas consumption, but how significant this result finally is, depends greatly on the engine and use, that means especially every unique application..