Hi
We had some tested a while ago, if I remember correctly and it was okay, it met the specs it claimed (cheaper oils don't always do that), but it didn't exceed the specs.
If you look at something like the Mobil ESP, it meets a lot of specifications. In order to do that, it will exceed the requirements of certain specs, to cover the others. For example, the 507 spec will mean that it's suitable for 20k/2 years and safe to use with the DPF, but by being C2 as well as C3 (the standard requirement for 507), in theory you can achieve better fuel economy.
Also, when it comes to basestocks, people assume that synthetic is synthetic and there are no variances in that. The top synthetics are group 5, ester oils, followed by group 4 PAO oils (proper, lab-made synthetics) and then the group 3 hydrocracked oils. Hydrocracking is a process that starts off with mineral oil, it's then heated, hydrogen is introduced, a reaction takes place and the oil is modified sufficiently that it is then sold as synthetic. There are different quality levels of hydrocracked basestocks, some are quite basic, some are really good and can be better than the PAO synthetics. Mobil 1 used to be a PAO oil, but it changed to a group 3 in the last few years. Someone tested a can of the old PAO oil against the new group 3 and the results showed the new oil was the better one. The thing is, those basestocks are only going to be used in the oil companies own oils, not in the oils that they are producing for other companies.
As well as using the basic option with basestocks and different additive packages to meet the different specifications, there is also the issue of quality control. One oil company tested about 20 different 5w-40 oils a couple of years ago (not ones that we deal with). About half of them were in grade, but others were not 5w-40s, one of them was a 20w-40 (almost a 20w-50). That was because quality control takes time and money, so when you are trying to produce a cheap oil, quality control is often something that is overlooked.
Cheers
Tim