I know I've posted on this forum with my surmizes that Mobil 1 0w30 and 0w40 may be the grades with the least VI improver added, with the wide viscosity spread coming primarily from the blend of high VI PAOs and esters in the base oil. Tonight, I decided to rethink that position while looking at the Red Line website. Briefly, here's why: Red Line oils use polyol ester basestocks which have naturally high inherent VIs and Red Line states plainly on their website that all their multi-weight grades are achieved without the use of VI improvers. But take a look at the VIs of the finished Red Line oils: they are lower than what you see in other finished synthetic oils of the same weight. Example: Red Line 10w30 has a VI of 137; Mobil 1 10w30 has a VI of 145; Amsoil 10w30 has a VI of 167. So, here's what I'm thinking: For Group IV/Group V synthetics, can the VI of the finished oil be and indicator of how much VI improver was used to obtain the viscosity spread, with a higher number indicating more VI improver and a lower number indicating less? My reasoning is based on the Red Line's VI being lower than both Mobil 1's and Amsoil's. The only Mobil 1 where Mobil states no VI improvers are used is the 20w50 motorcycle oil. The VI is 150. Red Line's 20w50 has a VI of 157. Amsoil's 20w50 has a VI of 160. Red Line's 10w40 has a VI of 159, while Amsoil's 10w40 has a VI of 183. So, once again it appears what we are seeing is that the oils with no VI improvers are the ones with lower VIs. (By comparison, one would assume Amsoil's straight 30wt synthetic would have no VI improvers, yet it has a VI of 124.) If you've followed my reasoning thus far, and applying it to the Mobil 1 line of oils, it would seem then that Mobil 1 10w30 could very well be the prime SuperSyn candidate for having NO VI improvers added. If the Mobil 1 20w50 has no VI improvers, and attains a 30wt spread with a VI of 150, then it seems reasonable to conclude that the 10w30, with a 20wt spread and a VI of 145 would be the other Mobil 1 with no VI improvers, especially in light of looking at the Red Line VI for the same weights. At any rate, I plan to call Mobil's tech line tomorrow and ask them directly which grades of the Mobil 1 have the least (or no) VI improvers.