I have seen 7 set ups with oil addition on Keystokers. The changeover is quick and fairly easy. HOWEVER, each of them have the oil burner sitting neatly beside the K-6's and K-8's, never used. These are larger homes, mostly 100 years old, and not updated with insulation or new windows. Those that are require a smaller boiler, like a KAA-2 a LL 110, A CoalJack 85K, EFM 350, an AA 130 loafing, a Van Wert 400....
Your question about efficency on oil is easily answered by the folks who manufacture them. I'm thinking around 75%, which is very good for a secondary fuel. On coal they are much higher.
The owners were soooo scared to trust coal they just HAD to have the oil as a backup. Some of these folks have had the units installed over 10 years.
Personally, I keep a separate oil boiler available but have only used it once, just to test it out. If you think the stoker will leave you stranded, guess again...the things are bulletproof.
If you think you can just flip a switch and fire it up, you are wrong once again. As Mick Jagger says: "You can't always get what you want !"
Don't think you will outsmart the old timers, just learn to accept things that you just plain don't understand.
Even in the coldest weather, the hoppers will last 3 to 4 days, more if you have auto fill augers....