Venting a new mid-efficiency gas boiler into a masonry chimney usually ends up with condensation issues due to the lower stack temp. If you right-size the boiler to the heat load (as you should) the odds are pretty good that the old chimney is too oversized to begin with. Almost all new non-condensing gas boiler installations require a chimney liner right-sized for the boiler's BTU output. What was the gallons per hour of the nozzle on your oil-burner?
For the record, it looks like about an 8" x 8" ? Or something bigger/smaller? About how tall (measured from the boiler's vent port to the chimney top)?
The vent diameter of the S-90 is 5", the smaller more appropriate sized boilers' vent is 4". The ideal diameter for the flue liner is almost certainly going to be close to the same size as the vent connection on the boiler, not more than an inch or so bigger. Since (from your other posting) it's a ranch house, the top of the chimney would probably be no more than 20' above the draft hood of the boiler, and you'll need the higher draft velocity of a minimum-diameter liner.
Edited to add:
On p.3 of
the installation manual it reads:
"
CHIMNEY RECOMMENDATIONS HEIGHT:15 ft. (minimum) from draft hood skirt to top of chimney. INSIDE DIAMETER: Same as dimension C (or larger). NOTE: Larger chimney may be required if two or more boilers or a boiler and another appliance are vented to a single chimney. Slant/Fin supplies vent dampers by several manufacturers. Some are smaller than the indicated “G maximum”.
So unless you have another big gas-burner using the flue you'd need a 4" liner for the smaller versions, a 5" for the S-90.
