Originally Posted By: SnakeOil
That said, I believe that if any motor oil claims it has any additional cleaning ability, it then has more detergent and dispersant content. In theory, a conventional could clean better than a synthetic oil.
Aniline point of the base fluids is a measure of their solvency of polar materials. Typically it goes as such:
PAO < Group III < Group II < alkylated napthalene < esters used in crankcase oils.
Lower aniline point = more solvency. About 2 years ago I had aniline point tests done on Mobil 1: 0W-40, Mobil 1 Turbo Diesel Truck 5w40, and 10w30 HM. They all had high aniline points and 10w30 HM had the lowest by a small but significant amount and Mobil 1 Turbo Diesel Truck 5w40 had the highest by a small but significant amount. That is, these oils do NOT contain highly solvent base oil mixtures. The cleaning they may do will be largely from the additive package.
Because of the range of possibilities depending on what's used, we can't generalize about base fluid solvency between conventional and synthetic motor oils without making big caveats. But I'm in agreement that detergent and dispersant additives play a big role and PCMO synthetics generally get the better and/or more concentrated versions of them compared to PCMO conventional oils.
In theory, a purely ester based oil with a large dose of the right type of detergent and dispersant additives would clean the quickest. The oil could cause issues with seals, however since that same solvency they have for polar materials also causes them to go after seal materials...it's "not all good". Red Line 15w40 HDEO is the closest thing to such an oil that I can think of but Dave at Red Line has said their oils use PAO along with the esters.