I suspect most of the fluids put into Chinese tractors - particularly Jinmas - is used stuff that was just put in there for warehousing and transport. Dealers that have an eye toward repeat customers usually remedy the mess prior to sale.
Can't put in used fuel though, so I suspect what you're seeing is mostly algae that propagated during transport and warehousing. Some of the tanks - for reasons that escape me - were painted inside as well. Even after you drain, algae will remain clinging to the tank walls, the float mechanism, and probably even the siphon. I'm sure your fuel filter is contaminated as well. Replace it, top up the tank with fresh fuel, and add a biocide. You'll continue to see dead algae accumulate in the sediment bowl, dump/rinse it once in a while. Even after flushing and all the above described steps, it still took ~9 months for that crap to stop accumulating in the KAMA sediment bowl.
//greg//