FWIW: Along with my engine rebuild I changed out the oil ZF6 and used Amsol's syncromesh. The trans had a "personality" that I knew how to work but with the Amsol it is very smooth, a nice improvement.

 

hopefully you werent one of the people using bmw engine oil in it ! (Per bad advice out there on the internet )

 

I am glad it worked for you. I prefer to use motor oil inside a manual transmission personally.

I know a lot of people today like a synchromesh fluid, and that might have some advantages when it comes to the synchronizers, but I think a motor oil is going to be a better lubricant for the dog gears, inuput/output gears and bearings

I've never read about using motor oil on the internet, I base my decision to use it on 40 years experience working with manual transmissions and that the vast majority of manual transmissions built over the last 100+ years rolled off the showroom floor with motor oil in the case.

 

I'm one of the People using the Bad Internet information BMW Oil in both my 96 GS and my 1990 Autocrosser.

 

I’ll stand behind my comments that bmw motor oil was a bad choice for MY ZF6

i want to first ask if you have swapped it out recently for new bmw motor oil transmission, or is this 25 yr old oil

i ask this question bc its possible the castrol stuff has changed or consolidated and the stuff that used to work (and be part of zfdoc’s recommendation) is no longer available

my zf6 has a blue tag remanufactured plaque on it. Thats the transmission i got in it in 2001 when i bought my 1993 with 26k miles on it

in 2002, i bought the bmw castrol 10w-60 oil and had i run it longer than 2 weeks, it would have destroyed my transmission

My experience went like this, the gear shift lever was immediately harder to shift. It liked to grind.

i took it to the dragstrip and had to abort every pass. The lever would jam and wouldnt move from 1st to 2nd at max rpm / WOT shift

i consulted with zfdoc and he recommended replacing/bleeding the clutch hydraulics, which I did and there was no change

he then recommended rebuilding the transmission (then 45k on it) at a cost of thousands

I replaced the fluid and noted lots of sparkles in it (from synchro material).

after putting gm syncromesh in it, all the shifting problems magically disappeared.

ive since replqced the fluid a couple times since then (i now use amsoil syncromesh) and i never see any sparkles in the fluid i drain out

Whoever had good luck (in thr past) with the bmw 10w-60 castrol motor oil , i challenge you to go out and change it out (with more bmw motor oil). Its possible something changed in the formulation or the stuff that used to also work well in transmissions is no longer available and what is available (as replacement substitute) no longer works in manual gearboxes

im not the only one here who has had problems

i believe zfdoc’s outdated advice stems from a time when the original factory fill zf6 fluid became discontinued and inferior alternatives were being recommended so someone could still be able to at least ‘drive the car to work’ etc, until proper fluids became available

i dont want to see anybody else waste money or possible ruin their transmission

 

I use 10W-30 oil because that is what is on the Champion lubricants MSDS sheet for the GM part number associated with the original equipment Part number (1052931). The GM part itself has been superseded for quite a while. I have not used a BMW oil.

I don't know that 10W-60 will be any better or any worse and I don't think the brand matters (and BMW does not make "BMW oil" anyway), but I think it would be fine.

As I noted above a sychromesh might work better for your synchronizers, especially if you are trying to force a shift at the wrong engine RPM and expecting the synchronizers to dramatically change the speed of the input shaft and the clutch disk, and even more so if your clutch is not disengaging completely or you are not keeping it disengaged until the shift is complete. But it you are rev matching and still can't put it into gear, that is a problem with your dog gears not your synchros IMO.

Even if your synchronizers are completely shot, you should still be able to shift gears by rev matching. I have not drag raced a manual transmission car and if there is a different way you shift while doing that and if that puts extreme load on the synchronizers then yeah, you would want something with more friction so you get less slip (and less wear) on the synchronizers.

I have 130k on my transission and it works fine (the engine is another matter )

 

my theory is the synchros need ‘some’ level of friction to engage

the castrol bmw m5 motor oil 10w-60 is too slippery

that zf6 handles 6500 rpm shifts just fine to this day, with the proper syncromesh lubricant

 

The RPM you shift at is not relevant. The sychronizers don't need to engage at all if you rev match your shift and put your engine where it needs to be for the gear you are selecting. Until the 1960s few manual transmissions even had synchronizers.

What the sychronizers do is change the speed (in the case of an upshift slow down) of the input shaft, clutch disk and intermediate shaft to what speed they need to be for the gear selected and the speed your vehicle is traveling. The further the engine is from where it needs to be with the new gear, the more the sychrnoizers have to work to "fix" this, and if you shift without the clutch fully disengaged, or you start to take your foot off of it before you are fully in gear, then they have to fight the clutch as well.

I get what you are saying and it is easier to shift with more friction, especially if the sychnronizers are worn. It also probably puts less wear on your sychronizers if you are slipping them less. But the oil's primary job is lubrication and millions upon millions of manual transmissions were built and shipped from the factory with a general purpose oil, including the ZF6.

Where modern sychromesh fluids really started to be used is after transmissions moved away from brass to carbon or sintered metal sycnchronizers.

 

My BMW Oil is pretty old. I bought mt 96 GS in 2006 and I bought twice to much oil for it. So when my 1990 Autocross car project car came to be last year I still had enough for it.