My real world experiences differ greatly from some theories put forth here, from four levels. 1. Owner/track driver/mechanic, 2. Vendor, 3. Engineer at the OEM level, and 4. Instructor and technical resource for PCA track events. I can list tons of vehicles that have used gear oil with a higher KV@100 than their factory fill, and successfully. In some cases, it is DRASTICALLY higher, such as guys who run 75w90 in their BMW's. This guy is one:
http://www.youtube.com/watch?v=QZaH1hxnBDs and his comments about the change were "completely transformed the shift feel of the transmission."
I cannot, however, find a single spec that calls out KV. SAE Viscosity and API GL-#? All day long. But not KV.
The Millers is a GL4/GL5 fluid. It meets GL4 specs (as well as GL5). It is a transmission fluid. As is their street version, which is labeled specifically as a transmission fluid....in a 75w90. And its KV@100 is even higher, at 17.85. Millers is not unique. Every one of Motul's 75w90 oils are GL4 and GL5. Royal Purple's 75w90 is a GL4 and GL5. Amsoil has an MT-1 and GL5.
Again, I cannot find an OEM who specifies a gear oil by KV. Furthermore, having been in the OEM product development world (used to do engine development for Ford), I can absolutely guarantee you that they are designed to accommodate the full range of the specification, and then some. You absolutely MUST design in robustness that the real world introduces. Case in point, albeit engine oil. I was involved in the early work that became the new 5.0L Coyote. The VCT system requires both oil temp data, and a positive feedback. Yes engine oils have many more variables, but there is still noise that must considered. When working with previously referenced race team, I had to get in touch with a former colleague who had D&R responsibility for the VCT system, since we don't offer a 5w50 (but do a 10w50). Sent him some data, and he verified for the team and me that the system is robust to the viscosity difference. And using oil as a working hydraulic fluid is certainly doing to put more demands on precise viscosity than a gearbox.
There are PLENTY of transmissions, NOT transaxles, that spec a 75w90, here are a few more that are higher volume ones:
- Mazda Miata, gen 1 (80w90)
- Mazda Miata, gen 2
- Mazda MX5 (current gen)
- Subaru BRZ/Scion FR-S
Which is, again, why you can go to any parts store, and find GL4 spec 75w90 sitting on a shelf, but you can't find the thinner stuff.
Some 75w90's that meet GL4 specs that have a higher than 15cP that may have been missed:
Motul Gear 300 - 15.2
Motul Gearbox 80w90 - 21.2
Motylgear - 16.7
Royal Purple 17.5
Red Line (GL4 only) 15.6
Mobil 1 Delvac - 15.9
Fuchs Titan Sintopoid - 15.2
Amsoil 80w90 (MT-1) - 16.0
Schaeffer 741 (MT-1) - 16.6
Torco SGO - 16.2
Torco RGO (80w90) - 20.2
Swepco 201 (MT-1 80w90) - 15.7
I stopped looking after that. And that doesn't even include the transaxles, which would be the majority of manual transmissions at this point in time.
Furthermore, about 30% of our customers have street/track cars. You'd struggle to get a gearbox to reach 200 degrees on the street. They'll hit 275 pretty easily on the track. If transmissions were sensitive to 1-2cP differences, then it would be impossible to run the same oil on the street and on the track (well, unless they were like damper oil w/ 350-ish VI's). But they do it. With virtually every oil. And as an nationally certified instructor with PCA, I get to interface with tons of folks who track their street driven cars. NEVER met anyone who had to change their fluid for driving on the track. The owner of the BMW listed above is one - his daily driver, he drag races it, and he also tracks it at Road Atlanta and Atlanta Motorsports Park.
Some other types of cars: tons of Porsches: 944's, 968's, Boxsters, 996, 997 (incl GT3). One of the 968's was mine. Caterham, VW, I forget others.
Probably 65% of our customers have dedicated track/race cars. Excepting a few BMW guys who run THICKER oil in their race cars due to heat, they all run the OEM spec viscosity. Which again due to heat will be notably different than 1-2cP from where the street driven spec originates.
That said, I can definitively say that it does NOT work well with Honda transaxles!!!! We had a customer with a Grand Am team running Civis, and the change to our 75w90 was disasterous. Hoped the higher temps would thin out the oil enough for use, but it didn't.
BTW, I should be able to make public a change that will be taking place for factory fills for one rather upscale vehicle starting in January. Currently has a service requirement of 40,000 miles. Will change to a single lifetime fill. The factory fill will be decidedly higher than 15cP.
As it is, I need to be productive the rest of the day. I don't mean to be argumentative, but like I said, the theories put forth do not align with my experiences, dating back to before I even knew squat about oil. We've already seen some confusion about the Millers in this thread, I just want to avoid more, since this one reads like a list of reasons to not use it.