I apologize in advance re the length of this post re manual transaxle fluids. I would like to get some discussion going to help me with the selection of a manual transmission fluid for my 3rd generation Mazda6 which has a 6 speed transaxle and has all of 1,200 miles on it. Not yet broken in, I know. The transaxle exhibits less than butter-smooth first and second gear synchro action. I note that this “Skyactiv” transmission has triple-cone synchro’s on 1st and 2nd and then single-cone synchro’s on the balance of the gears. Third through sixth gear synchro’s are totally smooth; 1st and 2nd are not perfect… exhibiting “double-bump” and some “notchiness”. In my view – seeing how often this comes up within the Mazda3, Mazda6, and CX3 communities – I would say that the transaxle design is “weak”. I can get totally smooth 1st to 2nd shifts by: i) double-clutching; or ii) by being really careful with the non-double-clutched shifts… shifts made only at pretty moderate road-speed-in gear speeds, by delaying the shift ever so slightly. The transaxle’s operation is a bit worse with cold MTF versus when at full operating temp. My goal is to smoothen-up first to second to the full extent that I can within the design limitations that (possibly) exist with the C66M-R transaxle. I have been the only person to have ever test-driven this car. I can, on an unqualified basis, say that the 1st and 2nd gear synchro’s have NOT been “knackered” as almost all of my down- as well as up-shifts have been double-clutched accurately. ‘Been at this for 42 years with no worn synchro’s. I want to try to use the best MTF I can, properly making the choice between shift-smoothness and gear- and bearing longevity. The mfr calls for GL-4 75W-80 fluid. Following, is a 40 or so year old statement by VW re competing considerations re the choice of lubricant. I believe that this may only in part be accurate in today’s world… in that specific synchromesh friction modifying additives have been added to varying degrees for some of today’s MTF’s – additives that don’t trade-away bearing and gear anti-wear properties in favor of better shift action. This is combined with lower viscosities at 100C, for both again-improved shiftability, and to enhance fuel efficiency. Still, I believe, feature-for-feature being equal, slightly higher viscosities will improve gear face and bearing life expectancies. Here's VW's statement: I am considering BG Synchroshift II and also am considering Redline MTF – both being GL-4 75W-80 fluids. For the best life expectancy I would in-a-flash choose Shell S6 GXME fluid, about 9.55 cS at 100C, as Shell lubricants are generally excellent, but I doubt that this Shell lubricant has many additives to enhance shiftability. They target heavy truck transportation and super-long OCI’s. I have not heard many disparaging comments re the Synchroshift II – but I note that its 100C viscosity is about 7.1 – really quite low. That gives me pause, at least a bit. The Redline MTF has a 100C viscosity of 10.4 – better – and apparently has a very good rep. re shiftability. It would seem to be a good choice, but I have found some rather disparaging comments about it. I provide these as images. I do not want to incite a highly negative reaction to these, but there does happen to be some negative info out there. These comments date from 2006 through to 2016: I would also consider other fluids…. combining wear-resistance and excellent shifting performance. Ideas? My questions include: -Can I feel confident that the Synchroshift II will give me fairly good antiwear properties, what with its so-low 100C viscosity; -How do folks answer the comment made that the Redline product is so full of additives, that it centrifuges out many of the additives, and leaves low viscosity depleted-additive oil to poorly protect gear-faces and bearings?; and - Should I NOT put rare-earth magnets, incorporated into both fill- and drain plugs, when I am planning to do a series of UOA’s? It seems the magnets would be best for after the initial UOA work. Thank you folks, in advance, for taking the time to comment.