Exactly. I've stopped using used engine oil on the bicycle chain long long time ago - it wasn't good. Used engine oil is good for fire and protecting wood on old fences, shacks etc.
He is a MECHANIC, not a chemist. Even if he didn't have any clue about oils, he could still be top notch with repairing/replacing bike parts. Old oil does become more acid like - so not the best thing for rubber.
What a load of twaddle "acid like" rubbish before engine oil got the levels of acidity as to be able to attack O or X rings the chain will have worn out due to normal levels of wear and tear. Service intervals are placed at levels where the oil is
still well in grade from some simple tests I did years ago you can almost double the intervals in normal operating conditions without any loss in performance. Engines since the mid 80's are massively better engineered, cleaner and dump less crap into their oil, yes in extremes it's possible but when was the last time anybody has seen oil sludge or highly carbonised oil during a normal oil change in a healthy engine........................never!
Any mechanic or persons working on engines and machinery worth their salt should know that, being a chemist has nothing to do with it, hence my comment.
We are going off at a tangent here the idea of using engine oil is to save money and use less oil in the bargain. Scott oil is good stuff that said it's bloody expensive standard Halfords 10W40 semi is £21.99 = £4.39 per litre Scottoil £13.88 per litre, remember this is a total loss system. Scottoil is approximately a £1 more expensive per litre than Halfords 5W40 fully synth
By using Scottoil you are chucking away 3 times the cost of using new 10W40 engine oil, by using old in grade oil you are saving all that money as you've already paid for it and using less oil so saving a vital resource!
As with everything in life it's a personnel choice if you want to spend your money on whatever that’s your choice and right but please don’t go using ridiculous arguments to support a view or try change the view of others.