This is cut & paste directly from the 18F252 data sheet:

2.1 Oscillator Types

The PIC18FXX2 can be operated in eight different Oscillator modes. The user can program
three configuration bits (FOSC2, FOSC1, and FOSC0) to select one of these eight modes:

1. LP Low Power Crystal
2. XT Crystal/Resonator
3. HS High Speed Crystal/Resonator
4. HS + PLL High Speed Crystal/Resonator with PLL enabled
5. RC External Resistor/Capacitor
6. RCIO External Resistor/Capacitor with I/O pin enabled
7. EC External Clock
8. ECIO External Clock with I/O pin enabled

Note there is no internal oscillator option for the 252. Oops...;o}