True synthetic oils are either category IV (Polyalphaolefin-((PAO)) based) or category V (polyester based). (Categoty III oils, such as Castrol, are really not synthetic oil but higher-refined petroleum oils.) A PAO will cause mild seal shrinkage, a polyester will cause slight seal swelling. Because of the seal shrinkage, manufacturers of category IV oils, such as Mobil 1, blend a smal amount of polyester to their predominately PAO-base stock.

So, if you wish to minimize seal leaks while continuing to use a synthetic oil, I would suggest you install the only category V (i.e., polyester basestock) synthetic available, Red Line Oil. Might just swell your seals enough to mitigate the problem.

I've used Red Line in my 240 for about 8 years and I change it once per year (supposed to be good for either 1 year or 18K miles). At 150K miles, the engine does not appear to leak or burn or consume ANY oil in a year's time. Never have to add any oil - dipstick is at full one year later. Search on ebay to find a retailer.

Hope this helps.

Augie