I don't have an 18F2520 to test this on, but give this a try;
Code:
asm
  __CONFIG _CONFIG1H, _OSC_INTIO67_1H & _FCMEN_OFF_1H & _IESO_OFF_1H
  __CONFIG _CONFIG2H, _WDT_OFF_2H & _WDTPS_512_2H
  __CONFIG _CONFIG3H, _CCP2MX_PORTC_3H & _PBADEN_OFF_3H & _LPT1OSC_OFF_3H & _MCLRE_OFF_3H
  __CONFIG _CONFIG4L, _STVREN_OFF_4L & _LVP_OFF_4L & _XINST_OFF_4L
endasm
        
DEFINE OSC 32     ' internal 8MHz x 4 with PLL enabled

blueled var LATB.4 ' prevents read-modify-write
redled  var LATB.5 ' problems

OSCTUNE = %0100000 ' turn on PLL for 4 x 8MHz
OSCCON = %01110000 ' set 8MHz internal osc
ADCON1 = %00001111 ' disable all A/D

main:
  low redled
  low blueled
  pause 500
  high redled
  pause 500
  high blueled
  pause 500
  goto main
  
  END