The first number refers to how the oil flows when it is very cold.
The second number refers to how the oil flows when at full engine temperature.
This methodology was needed for multi-weight dino oils as they thickened considerably when they get cold and loosened up a lot when they got hot.
T-6 doesn't thin out when it gets hot or thicken up when it gets cold very much at all because it is a group two catalytically created synthetic molecule that is created by refining tower sorting out a group 3 "slack wax" (a fairly consistent crude oil molecule that is huge but is very evenly constructed) then pumping the molten slack wax into a separate strongly agitated & heated/cooled tank containing a platinum cracking catalyst molecule.
This works by catalytically cracking that long long chain molecule into discrete chains of a very speciflc length that is controlled by the mating catalyst's molecular face that works by engages a chunk of the big boy slack wax molecule chain and breaking it off into very consistent length smaller chains. Grab and release happens at different temperature ranges, so the tank is hotter at the top and cooler at the bottom and agitator propellers force circulation in between the zones (and keep the heavier catalyst molecules stirred up and suspended in the moving soup so they can keep on grabbing, cracking and releasing)
Heating and stirring move the catalyst molecule from slack wax chain to slack wax chain, biting off the wax into a consistent oil length molecule that is of a length to be a very chemically stable free flowing oil. The catalyst can't grab one of the shorter chains that they break off unless it lines up exactly all the way across, so the catalyst generally just slaps onto a big long one and busts off another short oil type molecule.
After separating out the free flowing oil by settling and cooling, the catalyst settles to the bottom (which is a heavy metalic platinum class molecule that is so heavy that if not constantly agitated mechanically it would form a sludge at the bottom of the tank).
After about 20 hours of agitation the heat and cooling elements are turned off and the tank allowed to settle and cool completely. The unchanged long molecules of slack wax float up to the top and congeal into a solid and the catalyst settles down to the bottom of the tank and any shorter stub chains of slack wax float up to the top and congeal back into the solid wax mass. At room temperature the very very consistent length light brownish oil molecules remains liquid under the solid sheet of congealed wax and is pumped off to a separate tank.
This VERY CONSISTENT light brownish oil the base oil for Rotella T-6.
Since slack wax was considered a low value waste product (too sinky for candles or cosmetics) what of it that couldn't be sold as soft sticky toilet bowl wax "seating seals" was made into rubber tires or burned off in industrial furnaces and boilers or WAS THROWN AWAY before the catalyst cracking process was developed by Shell engineering.
The slack wax stuff was a pain in the butt to use as a fuel oil as it congealed in the pipes if you let it cool off, so all oil lines had to be insulated and heated with electrical tapes or steam sleeved pipes. Using it to make tire rubber had the same issues as the slack wax never got hard enough to bulk handle as a granular solid and using it as a heated liquid required all those troublesome heated tanks and shrouded steam heated lines again.
This "discards" origin means T-6 started out as a very very reasonably priced oil product ($13 a gallon). As it became popular and the demand skyrocketed as all us bikers suddenly discovered it, the price went up to $21 a gallon where it has remained.
Rotella T-6 is well understood as a very temperature stable oil. You can heat it up until it vaporises with a propane torch and it will cheerfully burn freely once lit but it will not get all ugly and chunky at the end like dino oil does. Try it, you will be amazed at the fact it burns until it is gone with very little greasy residue left over even when self-heated to 2,000
o F by the self-sustaining flame that is left when you remove the propane torch flame.
It does not form varnish or sludge by breaking down in your engine, which means it is very good for turbochargers, etc.
The Savage engine doesn't NEED T-6 unless the bike gets abused by lugging slowly up steep gravel roads or getting the hell run out of it then shutting the engine off immediately.
We generally could get away with white jug Rotella just fine ....
Those of us who use T-6 do it simply because we want to. Or we want to change our oil only once a year and feel completely safe by doing that.
T-6 and all the synthetic oils on our approved list have 60ppm of moly oil in them, moly oil which was added to make the synthetic oil be able to withstand very very long oil change intervals while cutting out most of the ZDDP which is required by oxygen sensors, etc. This moly oil is wasted on our bikes because our air filters are not good enought to keep out all the fine dust molecules that build up and show up as silica content during our oil analysis when we pay to have them done. Our air filter systems say we really do need to change our oil once a year to get rid of the silica, not because the oil itself couldn't go further. We never get to use the synthetic oil long enough for the moly additive do "do its thing" completely.
We are still watching that 60ppm of moly oil for any bad clutch effects (along with most of the other bike lists) as moly has been bad for clutches historically. Shell and the others claim that moly oil isn't the bad clutch stuff, so they call it "organic moly oil" to keep it separted in folks minds from the finely divided moly powders which is the bad sort of moly that got moly it's bad name.
White jug Rotella has no moly oil in it as it isn't extreme long drain interval oil. White jug Rotella is good for over twice as many miles as we put on our bikes in a year's time, but the ones who do run it long enough say it shears and begins to "lose oil" out past the rings long before it thickens or sludges .....
Which is why we tell folks to use the white jug Rotella if they get a energy star contaminated clutch as it does seem to clean up about 50% of the clutches when it gets used as a clean up agent. T-6 is too expensive to go into a short interval change clean up drill, so plain white jug Rotella is what the clean up folks should use.
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Why Rotella products? Very long use history by motorcycles of all brands with consistently good results, plus Shell actually rates the oil using approved JASO clutch slip testing methods.
There are some who say any good Heavy Duty Motor Oil carrying a C-I or C-J rating is good for the Savage. When these other things cost the same price as Rotella, I wouldn't bother as Rotella is the easiest to get of any of them. Walmart always has white and blue Rotella gallon jugs out on the shelves.
We have been running our Rotella white and blue jug recommendation now for over 5 years and we haven't seen any downsides. Rotella is the ONLY motorcycle oil that hasn't been abruptly reformulated by the maker to chase the ever changing (seems to be getting more and more NON-Savage appropriate) motorcycle oil requirements coming out of JASO. As far as JASO goes, right now I would flatly argue that the current crop of JASO MA-2 oils out there are
completely unuseable as a Savage oil due to the flat tappet/cam wear issue and very very low ZDDP amounts that exist with those MA-2 oils.
And what is flat scary is the old MA rating that we used to count on has been redefined as a "catch all" rating for oils that try for MA-2 and fail "only one or two" of the various MA-2 tests .... yup, you frick up and they'll give you an MA rating automatically. So I would also argue that MA is meaningless any more as it it practially undefined as far as anything Savage goes.
So we stick with what we know works .... Rotella white and blue jug.We understand it, we know how to bump it, we know we will be told by EVERYBODY if the formulation ever changes any at all (and so far it hasn't).
That makes Rotella a pretty good standard for the Savage motorcycle engine.
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