I've got some Amsoil leftover in a quart (about 3/4 full) and was curious if it lasts between oil changes or if I should dump it. I know brake fluid must be kept sealed because it absorbs moisture from the air, but I don't know about oil.

Thanks! It will only be left unsealed for like a month tops probably.

Keep it, it wont go bad in that short of time.. If it were 20 years i might think differently but to be honest where is anything going to go if its sealed?

Nah dude, don't dump it. Store it and use it if needed.