DIY BMW E39 M5 Thermostat 

I used a Stant 180F (82C) thermostat. Part # 14538
You need 2 different o-rings.
11531407002 x 2
11531406249 x 4
Good luck! It's a lot of tugging, wiggling and pulling.
Crush washer that I lost is part # 07119963072 x 2
Use some RTV Silicone for the Stant thermostat! The seals are slightly smaller and coolant will leak out when the system is pressurized. Very little RTV is needed, just around the rubber seal and some around the mating faces of the housings.