When faced with deteriorating engine oil pressure during a return to his departure airport an SR22T pilot learned an important lesson about declaring an emergency in a timely fashion in order to obtain the priority handling from ATC commensurate with his situation.

Narrative

Departed ZZZ for cross country flight VFR. Climbing through 6;000 FT the indicated oil pressure fell below 30 PSI. I immediately started a precautionary descent back to the airport and contacted Tower 10 miles northwest; then again at five miles. Oil pressure was in the low 20's by this time. Tower told me to turn right base. Oil pressure kept dropping. Turned final; my oil pressure dropped to seven PSI. Tower told a King Air to Line Up and Wait while I was on short final. Tower told me to execute a go-around; but by this time my oil pressure had dropped to two PSI and I was unsure about safely executing a go-around. I declared an emergency and the Tower cleared the King Air for departure. I landed behind the departing King Air. Clearing the runway I contacted the Tower and discussed the preceding actions. I realize now that I should have communicated my falling oil pressure at initial contact with the Tower; and perhaps declared an emergency much earlier in the process.