BMW uses a red colored o-ring to seal the interface between the engine block and the inlet pipe (part #18 in the diagram below)  but I'm not sure of the material composition of this oring.  For example, it's possible that the o-rings are Viton.




 have a metric oring selection kit with many sizes of black Buna-N o-rings. Buna-N is suitable for applications where the temperature range is 0-121 degrees C and has good resistance to petroleum-based oils, greases and hydraulic fluids, water and other common automotive chemicals.  Can I use a correctly sized  Buna-N o-ring in this application?

The black nitrile O rings will harden over time as you are geting towards the top end of the heat range where the red will remain softer and more pliable.
The nitrile will give probably 3-4 years service there where you can expect 10 from the red ones, they aren't viton but I cant remember what the stuff is called now. It will probably come back to me in the shower and then when I get out of the shower I will think what was that I had to remember.

I have only ever seen black Viton , and it is very expensive relative to other elastomers - and not really needed here. Most likely the original red one is a silicone rubber, designed for the high temperature.

I suppose they can make it any colour they wish. Here in Oz the only one I see is a light brown to identify it from nitrile.