Is synthetic worth the cost? Yes, synthetic lubricants are better. Yet non-synthetic lubricants are safe and effective. Good normal lubricants for diff and gearbox can last 20 or 30 years.

For best fuel economy, buy Energy Savings stamped lubricants. Synthetic blend engine oil is perhap your best economy. But then when regular oil is on sale, you can't beat the price.

Speaking of economy, is it good economy to save $3 on fuel over 3000 miles while costing $7 extra for full synthetic? Do the math. Manufacturers now specify more expensive oils. However they are under government mandate to reduce fleet fuel consumption whatever the cost. So we cannot assume anything by their adoption. They will pass the cost on to the buyer even if it cost more in the long run.