Are the effects of a conjured object or creature nonmagical?

Spells like Conjure Animals and Web can conjure creatures (a wolf) or objects (a sticky web). While the conjuration itself is magical (it was created by a spell), the effects of a conjuration are not necessarily magical. For example, this answer to “Do attacks from Conjure Animals creatures count as magical?” explains why an attack from a conjured owl is nonmagical.

According to the standard criteria, It seems like most effects caused by conjurations are nonmagical. For example, slipping on a pool of grease created by the Grease spell seems nonmagical:

  • Is it a magic item? No, it’s a pool of grease
  • Is it a spell? Or does it let you create the effects of a spell that’s mentioned in its description? No, it’s a pool of grease
  • Is it a spell attack? No, it’s a pool of grease
  • Is it fueled by the use of spell slots? No, it’s a pool of grease
  • Does its description say it’s magical? No, it’s a pool of grease

However, a similar argument could be applied to evocation spells, like Spirit Guardians and Wall of Fire. But intuitively, the effects a wall of fire (and most evocations spells) should be magical.

Are the effects of a creature or object created by conjuration magic always nonmagical? For example, does a creature with magic resistance get advantage on save against falling grease?

  • If so, does a similar argument apply to evocation spells?
  • If not, how do we determine which conjurations can have nonmagical effects?