[wolfi1]

I once had a problem with the ignition lock I couldn't turn the key, my mechanic told me that that could happen on a very hot day with that model. "use a lubricant or wait till it's colder" - "Would WD-40 do?" -"Guess so" made it worse. with the help of the AAA (well, the equivalent in my country) and an oil spray I could turn the key, since then I've always an oil spray with me

[anticodon]

Had the same problem with my moto (key not turning the lock). Fortunately, there was a car nearby and owner had a spare jug of oil. I put some oil on the key, put it in the ignition lock, waited for 5 minutes, and it started to turn again.

Although I must admin WD-40 helped me in the past opening an old door lock.

[regularfry]

I suspect the difference is whether (as with the old door lock) there is no lubricant at all and anything is better than nothing, or whether (as with the ignition key) there is a lubricant there which was designed for the purpose but for some other reason isn't working as intended, and which the WD-40 will displace and replace with something worse. "Fails in hot weather" sounds either like some sort of thermal expansion problem or the intended grease gets too thin to properly lubricate a high-pressure contact area. Or there just isn't enough of it.

[cucumber3732842]

You're not supposed to use lube on locks because the film strength of the oil will be enough to make tight pins that have tiny clearances not move.

Not really applicable in an automotive lock which start out as hotdog down hallway when new and only expand from there.

[lazide]

In both cases, the real issue is when the oil (eventually all do) oxidizes and ‘gums’. Tight tolerances make it cause worse problems sooner of course, but it’s the same problem eventually.

Putting new fresh oil in it often temporarily fixes it because it dissolves some (or a lot) of the old varnish. Acetone can often do the same thing too, but can also wash the varnish deeper into the mechanism where it turns into really solid ‘plastic’ when the acetone dissolves.

[anticodon]

I was 2000 km from home (1242 miles) and I was in panic because it was pretty uninhabited place. My bike is 12 years old but I used it in very harsh conditions (dirt, mountains).

Probably should replace the lock but it is so expensive.