When you say service it...what do you mean? what is the procedure to make sure you are fine. I haven't read anything about this and i have a oil pump in mine.
99% of the time, when people show up on this board, and say "I lost an engine, becasue the stupid oil injection failed!"... what they really mean to say is... "I have a 17 year old ski, and I never serviced the oil system, so I killed a perfectly good engine." (lol) Generally, what happes is... either the filter clogs up, the injection nozzle cloggs up, or the small hoses break, and fall off.
Ok, assuming it's been working... and you just want to make sure you are safe. What you do is:
1) Change the filter in the injection feed hose. (it's gravity feed, so a restriction will kill the engine)
2) Remove the small hoses, and make sure the nozzles in the manifold are clear.
3) Make sure the check valves at the pump are clear
4) Replace the small hoses.
On the 720 and smaller engine... it seems like the hoses are the main problem. Basically after +15 years... they just give up. Even if they feel soft, they are probably decayed. Give them a little twist, and they will probably rip. Needless to say, if that happens when you are out riding... you will loose an engine.
On the 800 and 951, the check valves can plug up easier. I think it has to do with the higher volume of oil. But here again... clean them every couple years, and you won't have an issue.
There are a few detailed threads on this.