Everyone's an expert
Recently my sister-in-law asked if it was OK to add conventional oil to an engine with synthetic in the crankcase. The counterman (NAPA) just about went nuts, and told her she could do extreme damage to the engine mixing the two. Right on the jug of Mobil 1 it says it is compatible with conventional oils - so who was right, the 18 year old high school "expert" or the corporation with extensive lab capabilities and lots to lose from a lawsuit? I've asked the same question and gotten answers all over the map: "It gums up the engine"; "It'll make the engine blow up" came from a couple of wet-behind-the-ears counter jocks. "No problem"; "I do it myself" from experienced mechanics and racers. I run Amsoil in two vehicles that each have about 160k, and I change it at about 1 year intervals, of course changing filters every 3k to 4k and topping up the oil as needed. The big advantage I see is in not generating all the waste oil and saving the time involved in oil changes (plus small mpg increases). The molecules in a good synthetic don't break down nearly as quickly as conventional petroleum oils - that's why it's guaranteed to meet specs at 35,000 miles, and why it's so good in gearboxes. Yep, if you have weak seals, it will find them and leak out - that's one reason it's better, it penetrates into smaller gaps. Yep, on engines that have sludge, it will dissolve it (it has a natural detergent effect) and on a real high mileage engine that has not been flushed it can cause the oil pump pickup or oil filter to plug up and starve the engine for lubrication. As in many other aspects of mechanical issues, you have to use your head. Every jet engine uses synthetic, because conventional oils aren't good enough. Two years ago I talked a local racer (track champ in Street Stock) into running Amsoil in his dirt track car - at the end of 16 races (same oil, never changed) the engine builder did a teardown and could not believe the condition of the internals, especially the bearings. He used the same bearings the next year - a hitherto unheard of procedure. Yes, some synthetics in the past have been "snake oils". But a good brand is still better than any conventional in the same weight, and if you can run a one year or more interval between changes, it does indeed come out ahead in a cost analysis. Blends are just that - a mix of both. The qualities and properties of the synthetic are degraded when blended - why do it, other than to save a few bucks? If you want to save money short term, buy the $2/qt stuff and change it frequently. If you want the ultimate in protection and performance, buy the best synthetic and install a by-pass filter system that will keep the oil super clean and allow 100k mi change intervals. Your money, your engine, you choose.
