Mark Smith,
I've been using Caterpillar's oil testing facilities, here in the Phoenix area. Costs about $18. 00/test.
In "MY OPINION, ONLY", you will get the biggest bang for your buck, using synthetic, if you do A LOT of driving, such as in towing commercially, etc. In that case, synthetics, in conjunction with bypass oil filtration, will allow you to safely extend your oil drain intervals. Severly cold conditions would be a good reason, also.
If you are still under warranty, you will have to change the oil at 15,000 mile intervals just to maintain the warranty, no matter what oil you use.
I've called Chevron & Schaeffer's Oil Co. & talked to their engineers. Both said to change the oil, annually, no matter how many miles are on the oil. That's probably a good idea if you do not use a bypass filter.
If you are not driving an extraordinary number of miles, per year, you may be better off just using a good conventional oil, such as Delo, Rotella, etc. , & change it more frequently.
I've run Delo to 9626 miles & the analysis came back indicating the oil was still in good shape. That oil change interval was just less than one year.
Keep in mind that there is the "warm, fuzzy feeling" you get by just using a better quality oil for your engine. That has 'value', also. Other than your wallet, you won't hurt anything by using a synthetic.
I remember reading on BITOG (bob is the oil guy.com) that Chevron Delo-400, 15X40, CI-4+, with the addition of Moly & Boron, was getting pretty close to the quality of a fully synthetic oil. Basically, quality conventional oils are pretty darned good & strong. You can't go wrong using them, either.
My reason for using UOA's (used oil analysis) was to be able to SAFELY extend oil drain intervals. Usually changed oil at 5000 mile intervals & had been reading that, due to the better quality oils, that schedule was somewhat premature. I did my first one at 7900 miles & the analysis came back good. Did the next one at 8300 miles & it came back good, also. The last one was at 9626 mile, with good results, also.
Good Luck.
Joe F. (Buffalo)