Too much overthinking

If you really want to service non serviceable parts with grease, than just buy some grease and "inject" it in there. It's not going to do anything good because the grease is just going to lay on top of the joint. The grease needs to be in between the two metal moving parts, not on top. That being said, you might have better luck using wd-40.

They are not considered life time either. If you go to real oem, you can find "service" items of said components. As seen here RealOEM.com - Online BMW Parts Catalog

I think 10-15 years of abuse is a great lifetime for a metal to metal joint.

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2001 E53 4.4 Alpine White, Sports Package
2000 E36/7 2.8 5 speed Bright Red w/ a HT!