I'm afraid I can't see the logic in using regular. In doing so, I would expect that you would induce more knock, not less, causing the knock sensor to retard timing even more. This should still make the engine knock as much as before, but perform worse. A knock sometimes indicates low flow in the EGR system, but my C36 doesn't have EGR. I have heard of MAF sensors becoming less accurate with time, caused by a slight buildup on the heated element. This will cause it to under-report the amount of air flow and cause a slight lean condition under certain circumstances, much as an intake leak downstream of the MAF would do, but I have no idea how to test the MAF sensor if it is not throwing up codes other than putting a new one in and seeing what happens.

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'96 C280 (gone)
'97 C36
'05 C230k