jtdc,
A fully synthetic oil means the base oil is synthetic while a semi-synth oil has a mineral base with synthetic additives- a full synth is better.

In theory, a full synth gives superior protection against engine wear since it circulates faster when cold and doesn't break down when stressed by "hot" temps.

The gas savings is achieved by using thinner viscosities, ie, 10w30 but these can break down if mineral based thus the move toward synthetics when using thinner viscosities.

Try a 5w40 or 15w50 full synth if your engine manual allows it.