This needs a full technical paper to properly answer.
Here's a brutally short story.
Magnets exposed to temperatures up to their Normal Operating Temp Limit are not affected by the heat.
Magnets exposed to a temperature beyond the Normal Limit, but below the Curie Limit, will lose strength, recover some of it after cooling, but experience a permanent loss of strength.
Magnets exposed to their Curie (temperature) Limit or above, lose 100 % of their strength, have zero recover after cooling, so in other words, are totally ruined.
In terms of a 919 magnetic trap, the rare earth magnetic material Neodymium would be fine for a magnetic trap bathed in sump oil, as long as the oil temp doesn't exceed 250 F or so.
Of course, if your 919 had an oil temp at the trap of 250 F, the magnet is the last thing to be concerned about !
Personally, if I was looking at magnetic trap drain bolts, I'd want a magnet type that would not lose any strength due to temperature range it was intended to be exposed to.