I find it rather interesting that a clamed bike mechanic recommends a non JASO MA2 engine oil for a bike with a wet clutch and gearbox lubricated by the engine oil.
If you have a bike with the wrong oil in the engine and the clutch sticks and you change the oil it will always improve. Almost entirely regardless of what oil you put in as long as it's just the correct viscosity.
The problem is your bike engine will make quick business out of a car engine oil (even your super fancy fully synthetic 10W60) and you will very quickly have reverted back to clunky gear changes and a sticky clutch.
For decades there was a lot of guesswork and try and error involved in selecting the correct engine oil for your bike. Motorcycle specific products were few and far between and no standards existed. This left you wondering if the expensive bike oil you bought was actually any different than the car oil which retailed for half the price on the next shelf. All the difference you had was a claim printed onto a container reading: Shear stable.

Then the Japanese gave us the JASO standards.
Using a non JASO MA2 engine oil in your bike since then seems medieval to me, sorry. Just like putting SAE40 into your airheads gear box just because you had it in your R25's box too.
Just my humble opinion.
In all your old air- and oilhead boxers you can obviously continue to use car engine oils happily. And please no SAE40 in the box. You can even run your R25's box on GL4 gearbox oil
