For floating lines:
I fill one 5 gal. bucket with warm soapy water, another with clear water. Strip fly line off into warm soapy water in big coils. Let sit a few minutes. Use soft towel, old sock, whatever, and strip the line through the cloth into the clear water bucket. On a clean floor, newspaper, whatever, strip the line out of the clear water bucket, wiping again as you go, onto the floor. Now, take yer bottle of Rio Agent X fly line cleaner and soak a sock or whatever with it. For this step it helps to have a peg on the wall or some other way to hang the line in coils. So, strip the line up off the floor, through the Agent X sock and loop it onto the peg/whatever. Let it sit for 24 hours. If the season is still on, back onto the reel it goes. If not, store in a warm dark place until the coming season.
I actually jumped through all these hoops before my September trip to the Miramichi... Awesome results for floating lines.
I have always...and likely always will..steer clear of automotive or any other product not specifically intended for fly lines.
A clean fly line is a happy fly line!