Can I use a 5v xtal osc with a 3v MCU? Will a simple npn buffer work?
1.1K/1.3K should work well. Vih is 0.7Vdd and Vil is 0.2Vdd for OSC input, so 
2.1V and 0.6V assuming a 3.0V nominal PIC Vdd. 

Here's what the waveform would look like with a 10pF load assuming a 35 ohm drive resistance from the oscillator. There is a nominal ~600mV
of noise margin on either side and the rise and fall times are not too terrible. 







simulate this circuit &ndash; Schematic created using CircuitLab