1989 yamaha 40 steam or vapor from exhaust only at low idle

Hi,
New yamaha owner here. 1989 40 hp precision blend oil injected short shaft, manual, pull start, tiller control.

Bought it from a outboard mechanic who said he fully serviced it including new water pump, tune-up etc.
Runs very well, pees very strong out of peehole, doesn't overheat, light doesn't come on, can put hand on block.

However, at dead low idle (which i troll at) produces a noticeable steam or vapor cloud out of exhaust. Is this normal?
Also don't know if this is related but seems to use a lot of fuel and very little oil. Only been out once so far and trolled
for about half a day and went through almost 6 gallons, this seems escessive for only trolling also when got home very
little oil gone out of oil tank. Is this normal also?

Engine runs beautifully otherwise.
Thanks very much,
tom

Note: Steam only happens at low idle, goes away as soon as speed up a little. Weird huh?
Thanks,
tom

Took it in to mechanic for repair, he just called me and says it has a blown head gasket and head cover gasket. He has parts on order.

Another fellow told me that once these are taken apart and reapaired, it is hit or miss whether they will be a good long-term engine. Is this true?
A co-worker here at the office who used to be an auto mechanic said no, there is no reason this can't be completely repaired to last if done properly.
Maybe it depends on the mechanic?

Any advise?
Thanks,
tom

No reason why a head gasket replacement won't last for years and years!!!---------Just too many " old wives tales " out there.------Make sure you see the old gaskets.--After all we see water vapour coming out the exhaust of cars on a cold morning ( closed cooling system ) and on an outboard, cooling water is expelled with the exhaust .------Then near the surface of a lake there is lots of moisture in the air and vapour from the exhaust will be visible on cooler days !!!

Thanks racerone,
i just got the engine back from the mechanic. He replaced the head gasket, head cover gasket, and thermostat, says he tested it thouroughly.
took it out for test run yesterday. it still steams at idle. mecahnic says it is ok.
i've seen enough outboard engines to know that this is not normal, the steam is excessive, it's rather embarassing to trail around so much steam everybody stares at it.
i think i'm going to give up and sell it as i don't trust it. If anybody wants it, it runs great and still lokks like new.
Any more advice?
Thanks,
tom