Manual Trans Oil

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Manual Transmission Oil

My 9/95-build 530i (stick shift) is equipped with a "ZF" 5-speed manual transmission, not the Getrag as suggested in the original press release.   The gear ratios are specified in your owner's manual, if you are interested.   The design is very conventional with the top gear being direct drive.  It's possible that the reverse gear has a synchro, if anyone knows for sure, kindly email me.
The gear shift lever is externally mounted and connects to the gearbox via a push-pull-rotating shaft.  Shift throws are rather long and the plastic shifter pivot bearing is known to wear quickly, aggravating the problem.  There are a number of aftermarket short-shift solutions available, the "BL/SS Level 3 with ERK" is considered the best engineered.  Be aware that shift effort will increase with the installation of one of these kits and in the case of the Level 2 kit, the transmission shifter shaft seal may wear out faster due to the increased shift rod angle.
Standard lubricant fill is petroleum "Dexron III" ATF (automatic transmission lubricant) which is preferred over "gear oil" due to its low viscosity, allowing easier shifting in cold climates combined with agreeable gear lubrication properties.
Without doubt it is worth changing this to a synthetic oil, the only question is which one.  My advice is this:  If you live in a warm climate use Redline MTL; if you live in a cold-moderate climate use Redline D4 ATF.  See Redlines website at http://www.redlineoil.com for an excellent discussion of these oils.  You will need 1.2 liters (1.3 quarts) obtained from a Redline distributor, and an appropriate hand pump available at most auto parts stores.
In my case, adding the MTL reduced a whining noise I was experiencing while in fifth gear.  The countershaft ("layshaft" in the UK and Australasia) is spinning unloaded in fifth and is free to vibrate within its drive gear tooth backlash.
Once you have synthetic oil installed, there is no particular reason to replace it on a regular basis as there is little degradation.  Typically gearboxes have a magnet glued to the inside floor to collect ferrous particles, which remain there for the life of the box.  However, being the picky people we are, change it anyway at 30,000 mile intervals.

How To Do The Job

Allow 1 hour to complete this easy job.  Normally it is recommended that the oil be warm before draining it but ATF is still pretty thin when cold.  Raise the the right side of the car in a safe manner (or park across a curb) and remove the filler plug first with a 17mm socket or box wrench.   Assuming it yields, remove the drain plug and allow 15 minutes for the old oil to drain.

gearbox_fill_plug.jpg (31490 bytes)  Filler Plug             gearbox_drain.jpg (36879 bytes)  Drain Plug

Replace the drain plug and pump in new oil until it pours out the filler hole.  The oil level must be set with the car level, so if it isn't currently level you need lower the car carefully and let it drip out and settle to the filler opening.  Raise the car again as required and replace the filler plug.
Bentley's quoted drain and filler plug torques at 37 ft-lbs are rather high for a tapered thread.  Excessive torque could crack the cast aluminum housing.  You can feel when the plug tightens - just add a touch more.  Check them both after a few days - and tighten slightly more if leaking is evident.  A tapered thread has very good resistance to loosening so don't fret about it dropping out just because you didn't apply a recommended torque.
I've noticed a bit more leakage with the synthetic oil, both on the Bimmer and on my Nissan truck.  There is not much you can do other than check the level every year or so.  Manual transmissions are very tolerant of low oil levels and it would have to be significantly low to damage the transmission.