I just replaced my control lines with new ones and I was trying to think of a way to help keep them like new. I thought, what if I'd spray them with WD-40? I've never put WD-40 on anything that it didn't make it work better. So, I took a can of WD-40 along to the field and sprayed the lines while they were on the reel before I unrolled it. Sure enough, the loops come out of the lines easier when walking the lines out. I don't like when there are too many twists in them and while walking them out the twists seem to not want to move towards the guy untwisting the handle and it seems to "curl" the cables. The guys walking the lines out also commented that the lines feel slipperier. I'm betting that the controls are working better later on in the flight as the twists in the cables accumulate.
I don't think it's going to do much positive for you, but it probably won't hurt anything long-term. I wouldn't hesitate to use it for a cleaner if I got something like uncured epoxy on the lines, or some gummy oil. It's a pretty effective cleaner for hydrocarbons so it will clean off any oil, and leave the metal wide open for rust. WD-40 won't prevent rust, but it also doesn't promote it, at least not directly. It was made for use on Stainless Steel in the first place, and stainless steel doesn't rust in any case. If you use on on music wire solids (not stainless like almost everybody in stunt uses) and leave it that way, you will remove all the oil and you will have reel full of orange corroded lines to throw away next time you go out.
After trying every treatment in the world (including those used by other stunt fliers successfully), I have pretty much settled on stranded lines, with lacquer thinner before every serious flight, every other practice flight, and NOTHING else. Occasionally I use a "flood method" of cleaning with lacquer thinner, basically running them through a puddle of lacquer thinner in my had and then very liberal flushing with a paper towel. If I had stainless solids I would clean with acetone between and immediately before each flight, lacquer thinner at the end of the day, and swapping them end-for-end after about 50 flights, then tossing them on the 100th flight (because the spalling off of metal makes them draggy out near the airplane end). If I ran music wire solids, I would clean them with WD-40 and then lacquer thinner at the start of a session, acetone between flights, and then something like air tool oil at the end of the day before rolling them up.
And it might be obvious (but apparently it wasn't given that I saw a 50-year veteran Top 5 flier do it with bad results) use a *clean* rag or paper towel to clean the lines! Only use it for lines, not wiping up fuel, etc. Aforementioned 50-year+ veteran was using an old rag, filthy, to clean his solids. He was complaining about them sticking, so I looked at his rag and then handed him a fresh paper towel to clean it again. Appalling black marks, took about 5 passes, half a bottle of lacquer thinner and 3 paper towels before it came clean. Magically, the lines no longer stuck, much to his apparent surprise.
BTW, Bounty paper towels, ideally the Select-A-Size type, are the preferred brand of towels.
Brett