MS & MX are aluminum body so no need for rust proofing.
Just picked up my M3 which is predominantly steel body with some aluminum. I plan on keeping my M3 for at least 20 years (I have a history of doing so with ICE so this EV should last well beyond that). My main concern is rust and living in Toronto, Ontario, Canada, the roads are heavily salted in winters. I've used KROWN rust control for many years and found them to be the best. My Toronto Tesla Service Ctr. rep stated not to rust proof due to potential for rust proofing solution to enter into the battery compartment. I would have thought the battery compartment would be completely sealed to prevent this. Looking for a second opinion from TMC contributors. I'll also be asking for 2nd opinion from Tesla.
PS. Love my M3.
Edit: KROWN is quite extensive. First visit has them drilling the car full of holes and filling with plugs in the quarter panels, wheel wells and other rust prone areas which allows their product to reach all areas of the car body.
I actually registered here just to answer you. There is only one underbody specialist in Ontario, it is Patrick Farray in Ottawa. Lots of people come from Toronto to do their car once,from Civics to 911 to million dollar cars, he has done them all. He has shown me lots of pictures of how Krown oil damages cars like yours or mine.
Your concern is valid- at 23% salinity (twice sea level on any de-icing operation in Canada), a one year exposure would eat several thousand microns, or 2-3 mm. especially brakes- two winter on any big brake kits (I get infinite with Cadmium plating, but it cannot be done to zinc coated rotors, which only lasts 2,000 hours or 3-4 months the first winter). The issue with M3 and any under-body that actually traps the salt water, and no car wash can remove it. Krown oil compounds the entrapment of salts UNLESS they open all your panels and know exactly how to spray your M, which they do not. 4th year when damage kicks in, owners come crying to Patrick. So on your M3 the most vulnerable part of the car is the subchassis, and trapped salt will rot it fast. Not as fast as a Civic with under panels today, but 4-5 years major damage.
tesla has another issue- unlike your bavarian, is cheap grade untreated steel, so it will rot (so say reports) in our climate esp Ottawa/Montreal a la Hyundai/Mazda. The bavarian chassis steel, high grade to commence, goes through miles of galvanization and prep- which T does not do at all, just paint. So when 20 year old e39s, which were not rust proof but high grade steel, go around immaculate today, your m3 should last as long or longer, provided the key vulnerability (subchassis and panels) is addressed.
In 5 years I did only two rustproofs with Fari, pristine under-chassis at 100,000 kms. Other people bring their cars, second winter, and indeed rust is already in.
Tesla folks are right about the oil, unfortunately. I would enter, and then oil starts softening (though not Ptrick's which is neutral) all rubbers and then salt gets in..