Plugged Egr Valves Why?

100K miles dealer said my egr valves were plugged. Said it was caused
by bad gas. He said it was common to some models of Honda. My
questions a 1. My oil consumption has increased by double in the
last 10k miles. 2. I have noticed an octane knock on acceleration.
That started about 10K miles ago. What's the chance the plugged egr
valves caused both problems? 3. I have mostly used regular octane gas
when the owners manual recommended premium. (98 prelude) Would that
cause the plugged egr valves? Dealer is charged $200 to repair. Cheap
compared to the $700 for the water pump and timing belt. I am not
complaining this is the first trip ever for services other then tires
and oils changes! thanks tom