i guess it makes sense to change the plugs, since they're cheap and have integrated o-rings.

the recommened fluid from bmw is:
Castrol SAF-XO 75w-90 for most 5-series
Castrol SAF-XJ 75w-140 for cars with limited slip (m5)

the problem is that you cant find this fluid in the usa. and my dealer only has it in drums.
i think you'd be alright picking any synthetic gear oil in the same weight.

i'm gonna try the lucas synthetic 75w-90. it supposedly has some of their additive to "stick" better.

 

I bought the Amsoil, 75w-90. This is better than Mobil-1.

 

my car has 85k miles, and my old fluid literally looked like new. i've never seen used diff oil look that clean.
i guess the factory diff fluid could literally be lifetime.

 

Just did the diff. oil change at 60k miles. Fluid looked a little bit "old" when comparing with the fresh Amsoil. Drained out 1 Qt 8oz, and filled in 1.5 Qt. I had to get a SK long 14mm hex bit socket to unscrew the fill plug. My 14mm snap-on hex bit was not long enough and was blocked by the rear frame right after the heat sink.

 

08 535i doesn't have drain plug. I'm planning to change the diff oil later but how can I change the diff oil?

 

Your drain plug should be #3 in this diagram.

 

Where is a fill plug? I think #3 is a fill plug.

 

#2 is the fill plug. The plug that is depicted in your picture is indeed the drain plug. If you look carefully up more on the differential housing cover, you should be able to see the drain plug. The location of the fill plug is the reason why you will need a rubber hose and pump to refill the diff fluids, after the drain process has been completed.

 

Left: 545i, Right: 535i



Maybe I'm wrong but I think #2 is not the fill plug.
According to realoem.com, there are 7 #2s, Hex blot with washer, for vehicles with Auto trans. 2 #2s: M10X25-10.9ZNS3 and 5 #2s: M10X60-10.9ZNS3. I think #2 is for the trans cover, not the fill plug.
#3 is located at almost the same spot as Rizbel's car, 545i. If #3 is the drain plug, I don't think it doesn't drain used diff oil completely. :thumbsdown:

 

While I don't think auto transmission fluid can last life long or engine oil can last 15k miles, I do believe manual transmission oil and differential oil can be life time oil with today's synthetic oil.

So, don't bother.