Typically I use the branded thinner with the specific acrylic paints. I recently picked up a bottle of the Ultimate Modeling Products (UMP) thinner from the UK, and some of their airbrush cleaner. I really like both of these products. I have done some testing with this thinner on various brands of paint, ie. AK, Ammo of Mig, Testors, Vallejo, and Tamiya. They all have worked just fine. Some of the paints need a little bit more thinner to paint and thankfully there is a "cheat sheet" from UMP on suggested thinning ratios. I have also used 91% ISO to thin some acrylics, dont use this for Vallejo though, you'll end up with a nasty snot-like mess.