imo I would switch to 87 since the engine was designed for it. Don't know what that 2nd owner was thinking :eh:

You could play it safe and use your 93 -> 89 -> 87 approach. Or go straight to 87 and see how the engine handles it and hope for no hesitation of some sorts. The ECU should adapt.


edit: The same case for my F4i. Previous owner used 93. Manual says 87. Switched to 87 and things have been just fine.