It could be a number of things really. I wouldn't expect bolt-ons causing lean/rich conditions to throw this code. The code is basically saying your wideband oxygen sensors (not the emissions ones blocked off in your testpipes, the front ones in the exhaust manifold/header) are responding too slowly relative to the fuel trims, which supposedly indicates a deteriorating oxygen sensor.
However, you're getting it in both banks and unless you've done something horrible to foul the sensors (leaded fuel, something like that?), that seems unlikely. It could be, as they said, the MAFs having problems, which could lead to problems with fuel trims, which could convince the ECU of O2 sensor issues. It shouldn't be from oil on the filters though, unless you've cleaned them yourself and applied way too much oil at the end of the process.
It could also be any number of simpler things, like an intake and/or vacuum leak somewhere.
The best clues will be to look back at the car's history leading up to the codes appearing. It takes a couple of drive cycles for a code like this to show up usually. What was the most recent work done on the car? Was anything done within a few days/drives of the codes appearing?
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