Mike, I did some research on it before I joined here, while still at bob's.
I knew the ****'s invented coal-to-gas and named Fischer-Tropsch process, and Shell found using methane as the feedstock instead of coal is similar, but without all the impurities found in coal.
This assumes the methane is properly reformed and scrubbed to remove naturally-occuring hydrogen sulfide gas H2S. So, GTL is far cleaner than a cat cracked, severely hydrotreated, Group III crude oil.
In addition, carbon chain saturation with hydrogen is significantly higher in GTL than from crude, this is why GTL base oil is a Group III 'Plus' - it is better than required for a Group III oil, but not quite as good as Group IV PAO.
So what? It means GTL base oil is less subject to oxidation than conventional crude Group III base oil, = lower varnish deposits over time, also lasts longer.
But it's still not as pure as a true Group IV PAO base oil constructed from ethylene gas via polymerization, the highest molecular saturation level possible - for a LOT more money than GTL oil.
I view GTL as poor man's Group IV PAO. Plenty good enough for the 5k mile OCI needed to dump high soot and fuel dilution contamination. PAO is a waste for such short OCI's.
Finally, the more saturated the base oil, the less ability it has to hold additives in solution, and the more polar ester base oil is needed to do so. GTL base oil needs more ester. Somewhere I read a pure Group IV PAO base oil needs 10% ester to be able to hold the additives in solution.
