I use an 'off the shelf' daily spay-in conditioner and you're right Anubi, it can make your hair go crusty after a few days.
Its just a build up that doesn't happen to real hair because real hair is constantly being supplied with natural oils.
Also your natural hair (at the sides and back) gets rubbed by your pillows at night and your topper remains un touched.
Try getting a soft 'paddle brush' and gently brushing your topper when the hair has gone crusty.
You'll find it breaks up the hair & re softens it and removes some of the product build-up.
Also, don't use the spray-in conditioner EVERY day.
You can use just a light mist of plain water and you'll find that reactivates the old conditioner.
Maybe use the conditioner every 3rd day and plain water in between.
And, as Paul says, when you do use the spray-in moisturiser, always lightly dampen the hair with water first.