I totally second the "paint thinner" idea.
I use Toulene.
It's a great, pure hydrocarbon, was used by F1 as fuel in the 80's when small V8's with big turbo's were making 750hp or more.
It's 114 octane, leaves zero residue, and strips off varnish and gums left over by petrol/gasoline you get at the pump. (Often not very clean stuff it turns out, and depending on the station, can have water, dirt, waxes and all sorts of stuff in there you do not want in your fuel tank, or engine.)
And all that stuff ends up being processed by the cats. Some of it gets trapped there, but the toulene leaves no deposits, and helps to burn off anything on the cats, cleans off excess carbon on your sensors, and in the process also cleans the entire fuel system of varnish and waxes too.
Hit up a paint store and they should be able to sell you a pretty good sized container for about what the automotive shops want for just a few ML/OZ of the same stuff in a fancy bottle.
You will want a long, thin funnel to use while putting this into your fuel tank. DO NOT leave this stuff on your paint if you spill it. It will strip even cured automotive paint if allowed to sit on the paint. It will strip un-cured paint upon contact. (So if someone were to "tag" your car with a spray can, you could take a rag with some toulene on it, and wipe the paint right off.)
It will take household type paints right off too. It's hard on furnature, so don't set the container down on anyting with a varnish that you want to keep.
(If some toulene dripped down the container outside, and you set it down on the kitchen table for example.. Your wife will not be pleased!) LOL
One more thing. It melts many sorts of plastic when it's undiluted. Obviously not the container plastic, but I've ruined pumps with Toulene, so if you pump it out of the container, you will want to "flush" the pump with oil to keep the pump seals/rubber flaps/pistons and stuff made from plastics intact.
I've learned the hard way that pumping toulene with the common plastic transfer pumps and not cleaning them is a one time only use deal. It will just melt the pump internals over time if left in the pump. (This got expensive when I used to run Toulene as race gas, and bought a fancy pump that screws into a 55 gallon drum. I used it once, was very happy, came back to use it again, and it was melted to the point where it was never going to work again. The vendor claimed it was rated for all chemicals, including toulene, but they would not give me my money back on a 60.00 pump either.)