Here's my question stated simply:
How long do boilers generally last? I'm trying to determine if the one in my
home will last through this coming heating season. Is there any way to check
besides looking for leaks?

Here are the details:
I've got a Utica oil-fired boiler with radiators, in a single family home.
It's a Utica O.U. Series. I'd guess the boiler is roughly 35 years old. The
burner on it was replaced, I think in 1995. The boiler also has copper tubes
running inside it to heat the home's hot water.

When a boiler fails, does it usually fail suddenly or can I limp through a
season with a small leak by simply refilling the water? It doesn't seem to
leak now, judging by the water level in the glass tube. I have to manually
operate a valve to add water, so I'd know if the water level was noticeably
declining.

What happens with a sudden big leak? Does the burner then just not come on
because of some overheat-sensor?

If it does develop a leak, can a leak be plugged?

There's one other consideration: I had it shut down from May to December of
2008. When I started it back up, all seemed fine but later I noticed a rust
colored stain on the concrete floor. I believe that happened because of the
sudden change in temperature. There has not been a single drop of visible
leakage since that one time. When I do empty the low water shutoff every other
week or so, the water that comes out is never is rusty. It is either black if
the boiler's been run a lot, or else it's pretty clear otherwise.

So, what should I do to try and figure if it will be good through this winter,
in PA? Thanks.