Part of the fun of open posting is expressing opposite views.
and much of the drama is for the OP to separate out what he likes and doesn't
Note to Larry : I didn't write you are wrong!
While most automotive type sealers fall into the rule posted above by Larry, R-6 is one of the few exceptions.
Its formula will not harm a system [ note to self: has not harmed anything I have correctly used it on] and the pump lube properties will help the system run smooth.
The added fact of the sealing quality - an insurance fund if you will - circulating all the time will pay back is less small leaks such as those pesky drips from globe valve packing. The sealer will also keep impellers clean but absolutely NOT plug a system [ when treated at the proper rates] . It doesn't have the O2 savaging elements of a true boiler treatment package, but the reaction of the sealer is to O2 exposure and in that, it will sacrifice some of its sealer . This turns into the crud you will find in the boiler mud legs. Wash it out occasionally, but understand that the fresh fill will need the o2 scavenged all over again.
Again you can over treat - This is where folks run into trouble- correct application will not mess your valves control's or traps - it will keep them clean - so a little is better than a lot.
R-6 is not bottled labeled or manufacture recommended for boiler use. PERIOD end of disclaimer!