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I'm not qualified to answer your question. Here's what I can say that's not speculation.
My K1600 service manual calls for BMW Synthetic OSP, which is a 75w-90 GL5 gear oil. BMW have a long history of using this specification in their automotive differentials. Although a motorcycle doesn't have a differential, it does have a hypoid gear set, so the environment is similar. My 2008 BMW manual just lists Castrol SAF-XO, which has the same specification. The final drive does not experience big temperature variations, so I just don't see the need to be concerned over anything beyond the requirement in the manual. I doubt that there will be any owner-discernable differences between brands that meet BMW's qualification.
 

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Discussion Starter · #22 ·
I don’t understand the qualification bit. I wasn’t asking a technical question, nor asking about the fluid itself.

In any event, I think your general posture is fair. As I’ve read and discovered more about BMW’s fluid recommendations, I do find it interesting that few to none of the alternatives I’ve found discussed here on the forum match what SAF-XO was, nor Syntrax is. They also don’t match what BMW specifies for their non-limited-slip final drives basically company-wide (including cars). I don’t know what reasons BMW may have had for choosing what they did. They basically chose a manual transmission fluid with a GL-5 level treat rate for the AW/EP additive(s), and they seem to be quite particular about not having the friction modification additive that makes a fluid suitable for clutch-type limited-slip diffs.
…or maybe they’ve changed that whole posture in recent years…? I don’t follow BMW’s new stuff.

As an aside, I believe these factors, though I don’t know the drivers behind them, are why Amsoil does not specify any of their fluids for the K16 (nor do several bike-specific fluid marketers). I can’t for the life of me figure out why Red Line recommends the Shockproof fluid.
 

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You raise an interesting question. My personal suspicion about the recommendation for non-limited-slip fluids was that they were concerned about the additives. Any fluid that is designed to wick into posi-traction clutch packs could likely get past the seals in bearings and into the grease. Earlier BMWs had sealed bearings in both the final drive and transmission. I think (not positive) that they've gone to oil-lubricated bearings in the 1600. I'd be curious to see a parts list for the FD and trans just to satisfy my curiosity. Non-LS gear oil is getting hard to find. Land Rover dealers sell it, and a Syntrax version is available in Europe. A few years ago I found SAF-XO at a supplier and bought enough to hopefully last the rest of my riding days.
I may poke around the Redline and Amsoil websites later on this morning after my turkey goes on the smoker!
 

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Discussion Starter · #24 ·
I think Red Line's 75W90NS is the appropriate choice from their line. That's much like their 'normal' 75W90 fluid, but without the limited-slip additive. However, the very significantly lower Brookfield viscosity of that fluid may indicate a different base oil mix between the two (or maybe it means something else, but I sincerely doubt the elimination of the limited-slip friction modifier is the only difference).

Amsoil's 80W90 gear oil is the closest I can find from them, viscometrically. That's not a 75W90, though, of course. None of their manual transmission fluids are GL-5, so even though the one may work viscometrically it's definitely NOT suitable for the final drive on the K16.

Lubrication Engineers' 75W90 Monolec Syn Gear Oil is right on the money, viscometrically, and everything of theirs I ever used was excellent. It is suitable for limited-slip diffs, which again I can't tell whether that matters for the K16 final drive. I can't imagine it makes the slightest difference, but don't know. Also, I just don't need a 35-lb pail (aka 5-gallon bucket) any more. LOL!
 

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Amsoil Severe Gear 75W-90 does not meet the K1600 requirements? It’s all I’ve ran for 59,000 miles. Every time I drain it it looks like brand new fluid. Can’t say I ever dug too deep into the exact requirements…
 

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Discussion Starter · #26 ·
I’m trying (perhaps unsuccessfully) to phrase myself so as to be clear in saying that what Castrol’s SAF-XO was when it existed, and what Castrol Syntrax LongLife (the replacement for SAF-XO) is now, is significantly different from most of the alternative fluids discussed on the forum here, including Amsoil’s 75W90. The differences are both viscometric (Brookfield dynamic viscosity in particular) and in additive composition (no limited-slip additive in SAF-XO).

I can’t say whether that matters in the K16. I don’t even know whether SAF-XO’s physical properties and composition were specified (required) by BMW for the K16 final drive or not. I simply find it curious that these aspects of final drive fluids are generally different from what is usually discussed here, and are apparently(?) generally ignored.

IMO, a ‘good’ non-limited-slip final drive design shouldn’t be sensitive to those things, but I’m positive there were many considerations in play when the fluid was chosen. Maybe one or some of the folks here have knowledge I don’t that’s particular to this drive or fluid choice or something, and so it’s perfectly reasonable and fine to not worry about those things. The fluids many here choose may be BETTER than BMW’s fluid. IDK, and I can’t find data upon which to make an empirical choice.
 

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Discussion Starter · #28 ·
That would indeed be simple, if that's all they spec. My owner's manual doesn't say anything about the final drive fluid, that I've found yet. (Annoyingly, it actually says almost nothing about service or maintenance of any kind at all, including what is supposed to be done, when). The service manual copy I found linked here, or maybe from another forum site, says:
"
Technical data
Bevel gears, oil capacityoil change180 ml
Fluids and lubricants
Castrol SAF-XO
"
(emphasis mine)

So the only reference that I have says that BMW specifies that one very specific fluid. It is from there that I have moved in the direction I've moved re: what qualities to look for (since SAF-XO doesn't exist any more).

I'd be very happy to have a newer service manual, particularly if it gives updated information.

It's all academic, of course, since I changed the final drive fluid the other day. I had a bottle of Castrol Syntrax LongLife delivered.
 

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Had no idea there was so much to know about differential lube. @KL51212 , has done his homework. I'm just glad his obsession is lube oil and not tires.... :) :) :)

Duane
 
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SAF-OX is too available. I did a search for it and it's on eBay, Sun Coast Cycle Sports, and bimmerworld.com. I bought some early this year when I was going to change the oils. Liter bottles and 7 oz bottles are on eBay. bimmerworld.com says its for BMW's (cars/SAVs) up to 7/2011 production date without LS differentials.

Now, about tires! Dunlop Road Smart III's.
 
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