i think mboulais has the best advice in this thread so far.
oil is changing a LOT these days. there is a great deal of technology and it is all bound up in the api classification codes and mysterios additive pacs that are proprietary and known by no one - not even the engineers who developed them. just some old guy at the plant named bob who actually ads them to the vat and he is blindfolded en route to the facility.
the classification codes are what the typical consumer should follow for the best results. if you want to get obsessive,
Bob Is The Oil Guy has all the gory details, user testing, lab analysis, etc. anybody could want.
a good example of this is the legislation around synthetic oil and what characterises a full syn lube. several manufacturers have been criticized for bending the rules because their full syn is actually a blend coming from a different base oil group (III rather than IV). recent court case allowed some creative marketing, so now most full syn labeled oils are actually dino blended synthetics or multi base blends. only a handful (royal purple, redline, amsoil, etc.) are actually 100% synthetic before the additive pac is added. and you'll pay a pretty penny for the purity.
writing of additive pacs, there is another area of high tech wizardry. again, legislation has forced confusion. want the zinc and other additives that make your valves last longer in your gas engine? buy mobil 1 truck and suv. it's regular mobil 1 that has the additive pac which was outlawed for regular autos due to emissions, but over the road truckers started experiencing high mileage valve train failures. it was traced to the additive packs missing additives. so if the oil is branded for "truck" use, its allowed to contain the additives outlawed for the regular auto user.
some folks swear by the german made castrol known as GC gold. it is a true synthetic and is the only syntec made in germany. somehow, it manages to turn in some of the best engine wear analysis numbers from any oil. some attritubute it to elves, gummi bear additives or green beer used in the manufacturing process.
oil selection is a complex topic and you can't just do what your dad or grandfather taught you when you were younger. it's virtually impossible for the average person to keep up with this technology.
follow the instructions of the engineers who developed your engine and wrote the manual. use the vicosity recommended by them and make sure your "brand" meets or exceeds the classification code specified.
If anything, I would have expected the opposite. I guess that until Shell reformulates RotellaT synthetic, you might be better off not using it with ULSD.
