[KTL]

You didn't say whether or not you drained the oil. If the oil level is over-filled, you will have an intake manifold filled with oil and that can quickly foul your plugs. The backfires can be what is contaminating the MAF because the oil overfill can be spit through the oil tank vent hose, which incidentally is connected to the rubber elbow on the throttle body. So it is being spit right back into the intake system!

I would clean up the MAF as best as possible and also remove the bottom "waffle plate" to see if the wiper contact board is contaminated with that grey goop crap you pictured.