[CZARCAR (OP)]

which i fire every 2 days for hot water. As is, I can block air intake thru the oilgun at the bottom so the unit is kinda air sealed. By adding a damper above, I can close it so to block the openness of the fluepipe some. Would the added restriction of the damper help keep the boiler from losing heat significantly?.........thanx
 damper gets closed after gun stops firing.

[CliffordK]

Depending on how your chimney is setup, a damper might help a little bit.  However, I think it would be risky. 

You would certainly want to have an electronically controlled damper. 

If somehow you managed to get the damper closed while the stove was burning, you could risk carbon monoxide poisoning.  I don't think the oil fired stoves burn quite as cleanly as methane powered stoves which could potentially be run unvented, but you would still probably want to vent if possible.