let's trim the above quote
If a 4MHz ... in 10us increments.

If a 20MHz ... 2us resolution.
So if you have '100' @ 4Mhz... you'll have '200' @ 8MHZ. Of course it's linear as the OSC definition don't change anything in the result.

It doesn't matter if you use the internal or the external one... must have some heartbeat somewher