No, DEFINE OSC 8 tells PBP that the chip is running 8Mhz. It uses this
value to set the length of all its software timing loops. This does NOT
cause the chip to run at 8Mhz.

You need to define the oscillator type in the configuration fuses. Look at the
oscillator configurations in the datasheet for your processor, and then do
a search on "configuration bits" in this forum.