Good info Mike ~ thanks for digging that up & posting it.

ORR ~ DeanB

On 28 Aug , 2012, at 6:21 AM, Mike S wrote:

> On 8/28/2012 3:10 AM, Gene P wrote: > >> 1) The German mechanic that installed a rebuilt transaxle for me 9 >> years ago said "use GL-4 oil only, NOT GL-4/GL-5, GL-4 ONLY. Wrote >> it that way on the invoice. 5) A clip from a dandy 2-page technical >> article on Redline's website: "GL-5 gears oils which are required in >> hypoid differentials are not used in most synchromesh transmissions >> because the chemicals used to provide the extreme pressure >> protection can be corrosive to synchronizers, which are commonly made >> of brass or bronze. Typically, the use of a GL-5 lubricant in a >> synchromesh transmission will shorten the synchronizer life by one >> half." > > This thread started with someone saying "don't use Redline MT-90..." > based on a forwarded message where a friend of some person was talking > to some other, unnamed person at Redline. Redline themselves says, in > writing, about MT-90 "Popular in ... VW/Audi ... Safe for brass > synchros > ... Recommended for GL-1, GL-3, and GL-4 applications." That's us. > Where > it is not suitable, and they don't claim it is, is for a GL-5 > application. Redline recommends their 75W90NS gear oil for that, in > the > MTL/MT-90 spec sheet. It's odd that the forwarded "friend of a friend > told me" post was saying "use the GL-5, not the GL-4," the exact > opposite of what you usually see. > > Again, the GL-5 comments above don't apply to Redline MT-90, at all. > But, regarding those comments, I believe this is where a lot of > confusion comes from. GL-5 has greater extreme pressure lubricant > requirements. Early formulations achieved that by using sulfur > compounds > which corrode yellow metals (synchros). They tend to smell really > nasty. > > So, you will find people making blanket statements like "never use > GL-5," because they're ignorant of the reasoning behind it. It's not a > problem with GL-5, per se, but with _some_ GL-5 formulations. Many > newer > GL-5 formulas use additive packages which are no more corrosive than > GL-4. > > GL-4 corrosion requirements are measured using a ASTM D-130 copper > strip > corrosion test. GL-5 does not have to meet this test. ASTM D-130 is > designed to assess the relative degree of corrosivity of the sulfur > compounds contained in a petroleum product. The test is carried out by > immersing a polished copper strip in a given quantity of sample, > heating > at a temperature and for a time characteristic of the material and > comparing the copper strip after it has been washed and dried with the > ASTM Copper Strip Corrosion Standards. This produces a rating, where > 1a=best, 4c=worst. > > Although GL-5 doesn't have to meet the test, the MT-1 spec (and > MIL-PRF-2105E & SAE J2360) does require a D-130 test. The MT-1 > requirement is actually more stringent than the one for GL-4. The GL-4 > requirement is 3b after 1 hour, the MT-1 requirement is 2a after 3 > hours. > > So, if you're going to use a GL-5, it should _also_ carry one of these > ratings. Such oils are not difficult to find. > > The other reason you may not want a particular GL-5 is shifting. > There's > a range of properties, etc. which fall into the 80W(-90) weight > rating. > Some oils are affected by temperature more than others. Some > transmissions have a different shift feel than others. Various > combinations produce different results. But, the same applies for GL-4 > oils, some just work better than others. I have MT-90 in mine, and it > works for me. I know there are lots people who use it in newer VW/ > Audis, > because it works for them. > > If you Google "MT-1 ASTM D130", you'll find lots of info (and some > mis-info).