Article by: Paul Axford
Article applies to: most legacy BMWs
According to the manual 1.7 liters is needed. Information on the web indicates the correct lubricant is Castrol SAF-XJ (inc LSD) or as a second choice any synthetic 75W-90 GL5 gear lube. Requests locally for the Castrol just produced blank looks so I settled on the only synthetic hypoid gear oil available, Redline SAE 75W-90 GL-5+, luckily the same viscosity spec. The local auto parts store stocked this and gave me a substantial discount when I asked, down to NZ$22.50 a liter. Still ridiculous for oil that sells for about US$7 a liter you-know-where.
A look underneath indicates that my 730’s drain and fill plugs require a 10mm hex key to remove, not a 14mm as I have seen commonly quoted for the 735i model. The fill plug has limited clearance so the socket type won’t fit with the drive handle attached.
I ending up doing the job using a 3/8″ hex key, which worked perfectly. A cheap petrol squeeze-type hand pump from Repco was ideal for installing the new oil. Be aware the drain and filler plugs are identical and both have aluminum gasket washers.
The old oil was so clean I realized I didn’t really need to change it. If you don’t have an LSD, I would suggest you don’t bother. All wear particles are ferrous and will stick to the internal magnet, which I haven’t seen but assume is present.
Pro-Tip: When replacing the fluid, make sure that you remove the FILL port plug FIRST. This way you’re not stuck with a differential that’s been emptied . . . that you can’t fill. This would be bad. ~ Mike Oswald