Two questions:
1) Treadmill oil seems somewhat more expensive than d-i-y type Silicone Oil. Are there differences, eg alcohol content? or is it just a way to make more money?2) My spray can of Baufix Silicone oil says it's ok to 12/2018. What harm may be done in using it now?
obviously the latter
none. If oil went off in short order in the can it wouldn't be much use. People have been using 1950s (petroleum derived) engine oil & finding it still as good as it was, though the modern stuff performs a bit better. 1930s engine oil was castor, and is unlikely to be in a good state today, plus it performs badly by modern standards.
But it smells great as the car goes by!
I think castor oil - apart from racing - had disappeared in the 1930s. Mineral oil was first produced in the 1860s, and just kept getting better.
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