Er....I think Mickey-G is drinking the gas.....
87 octane fuel is perfect for running a mower on. Most of the world uses 95 to 98 octane fuel, as the liquid cheese that passes for fuel in the States is not available elsewhere.
In addition, a hotter plug will cause pre-ignition due to the reduced anti-knock properties of lower octane fuel (that's what the octane rating means - higher numbers = more knock resistance).
If you really want to use a lower octane fuel, put in another head gasket (meaning fit 2), and run on kerosene (yes, I am serious). You would need to rig a duel-fuel tank, so that you start up and shut down on gas, and run on kero. Some older Briggs engines were built this way.
In answer to your original question, yes you are being ripped off. Assuming the original diagnosis of fuel in the oil is correct, the problem needs to be fixed, not simply masked by replacing the oil and filter umpteen times and charging for labour each time. I think John has it right with point '1' in his answer.
phil_saunders(a)bigfoot.com
Phil Saunders
July 2009