I'm pretty sure that Sennheiser pretty much said that it shouldn't make any difference. What does make a real difference is the pads breaking in and adjusting to your head shape. Also headband clamp loosening etc.
Car engine comparisons do not apply headphones or electronics for that matter. Manufacturers do not say that you are supposed to limit fps for the first 30 days...
Yup. Sennheiser said that so I would follow that here.
Blanket statements about burn in are pointless. Some manufacturers do recommend burn in for particular models.