With the properly chosen base-stocks and additives, the only advantage of a synthetic really is extended drain-intervals.

My mechanic, whom I forced to fill my tranny with SWEPCO ATF, was absolutely amazed when he drained the fluid 30K miles later.

The fluid was so clean and red he thought it was new, and the tranny valve-body was so extremely clean. Even the tranny filter showed no particulates, and he can attest to hard my shifting is on both the 500E and C36-powered wagon.

The same was true wth the SWEPCO 201 80w90 gear/diff oil. The fluid came out SO CLEAN 30K miles later.

My mechanic also races SCCA SPEC-7 RX-7's and Improved-Stock Subaru's. He's now a believer and ONLY uses SWEPCO products.

Rick Ellington of RC-Imports who races MB's, had a diff that lost a fill-plug, and they continued to run at less than full-capacity for the duration of the race.

He tore-apart the diff later, and measured no wear! Rick is the one who told me to use SWEPCO lubricants, esp. since I purchased a 3.07 LSD from him.

:-) neil

BTW: I'm always on the lookout for better products, and I'm going to give Schaeffer Lubricants a try. Industrial and agricultural users swears by them, as do some racing teams. While Schaeffers does provide a 100% synthetic, they don't heavily promote it, because their blends also use such good base-stocks and additives and have proven themselves in engine and gearbox oil-analysis, and they (blends) cost less than 1/2 of their 100% synthetic-line.