Cast Iron blowers should be kept as clean as possible, with minimal corrosion. The saving grace that many users benefit by is something called "high-heat clearances". This a special modification that some TM OEM's specify wherein the blower lobes are shaved down a bit to create larger than stock clearances. This is done because some OEM's, especially those using blower exchangers, with often tax a blower to elevated temperatures that can heat-seize or actually heat-knock the blower due to heat expansion. The additional clearances permit that expansion, whilst maintaining adequate clearances at these high heat levels. This is why we see a bit less of the what I used to call "Holiday 3-day weekend blower seizures", when we'd always have at least one person in our service department with a stuck blower after a long holiday weekend, where a wet blower was allowed to sit long enough for rust to perform its "magic". Most of the time, daily running is enough to knock most of the rust off of blowers that receive less than desirable maintenance on the daily blower lubing regimen.
Still, it makes sense to protect the blower lobes from corrosion, which also attracts other build-up over time. Your blower bearings should outlast the rest of the machine, unless you give reason for strain induced wear, not to mention the occasional lobe seizure on occasion.
Too much, or worse, wrong stuff can have it's own set of negative consequences. Downstream exchangers with high-efficiency thin fins can gather dust very well, and coating them with sticky lubricant is a prescription for build-up cause clogging. Many owner manuals give brief, cursory statements on daily spray of a "lubricant" with simple instructions like "spray ten seconds at the end of the day, something like WD-40". Those bother me, because they over-simply the situation. No warning is given for the ramifications of spraying too much, or even some direction on what is being accomplished. In my opinion, it is by far a better idea to use a roduct specifically designed for corrosion protection (i.e. Corrosion X or Corrosion Block), and a good one, so we can cut the spray time in half, (quantity) far reducing the chance that we'll be also coating the TM's heat exchanger with goo.
