EDIT - Ignore everything I said. Didn't realize you don't even ave a clay tile liner. Yes you need that fixed.
Our 83% gas boiler is vented into a 15 year old clay lined flue. We got differing opinions as well.
Sweep says must be lined with stainless (he gets $$$).
The boiler tech says that lining is required by code but unless its a condensing unit that clay liner is OK. He also said that the acidic erosion issue is mostly an issue with gas units because of the lower flue gas temps and higher amount of water vapor in gas exhaust. Metal liners for gas must be stainless, but apparently oil units can use a plain steel liner, similar to woodstoves.
Or was it the sweep that said don't bother and the boiler guy said do?... Hmm don't remember exactly... But the bottom line is hat the boiler was installed in 2005 by the gas company and passed inspection so I imagine what they did was code legal at least in 05.
BTW, if your unit was a condensing furnace, it would have a power vent, usually piped into PVC directly outside, along with a dedicated outside air intake. The fact that they vented it into an existing chimney tells me that its likely a natural draft appliance and probably below 90% efficient. Lining cant hurt but may not be the dire emergency you suspect.
Go over to the "wall" forum at heatinghelp.com. Some really good HVAC pro's there who can help.http://www.heatinghelp.com/Forum