Years ago, when the car manufacturers started specifying 10W30 over 10W40, they claimed it was because of the number of engines they found sludged up, and it was because 10W40 had over 25% additives. Maybe this was just a cover to get lower viscosity hours adopted to get better gas mileage? It seems to me that 5W30 oils would also require more additives (VI Improvers, whatever) to cover the range. Now I see where many here on this site recommend 15W50 or some other higher range of vis, and I would think that would also require quite a bit of additives to do this? Does anyone else share my concerns - or can someone educate me on this subject?