For really blasting the carbon and varnish off your valves, see if you can't find a shop that uses Motorvac/Carbonclean products. It's not a gas tank fuel additive, a service dept. will use a machine and pump it straight into your engine. It is much more concentrated and does a better job of cleaning the entire cumbustion chamber. Of course, it is also more expensive - maybe about $100 in the US. I'm not sure, but I think they sell in UAE. Check their website motorvac.com for customer service.
After that, you could use a fuel tank additive for preventative maintenance.
After that, you could use a fuel tank additive for preventative maintenance.
