Correct...assume any posted FPS limits are measured with 0.20g BBs.
You bring your own ammo to most every game. Even if you're going to shoot 0.25/0.28/0.30/etc...if the host wants to chrony every gun, then they'll usually provide 0.20's (if they're smart) for enough shots to measure (just need 5-10 per rifle).
Some games they're not so well prepared and so guestimate what a XXX fps rifle shooting 0.25's will shoot with 0.20's. Kind of half assed if you ask me. Some will have a grid/table of what FPS is the max for what BB weight....smart idea, but I've only seen this used once.
Some games (very few that I know of) will require you to use the fields ammo. This may be because of stringent conditions or peace of mind for the land owner. They'll typically require you to use their biodegradable ammo...that they've cleared with the land owner. These games are the minority.
So...simply...measure/confirm your FPS with 0.20's...then there's no guessing at what it shoots. Bring whatever BB weight you want to use to the game. Listen closely to the host...abide by whatever conditions they and the land owner require you to adhere to.