Thinner into WMO or WVO would probably work really well as a viscosity adjuster. I think I'd be careful about too much thinner with high outdoor temps-explosion hazard in the tank - so save the stuff up until fall, then use it to thin your too-heavy fuel. Likewise, avoid adjusting the viscosity too thin - If you run a body shop, you know what a cup viscometer is. Make one out of a can or something and time #2 diesel running through it, then get your fuel blends to close to the same number.
As for chemical compatibility, the big worry is generally leaving the stuff sitting in the lines. As somebody earlier said, if you run the stuff out and replace with pump diesel before parking it for three months, you avoid a lot of the possible problems with thinner either eating parts of the fuel system or having some unpleasant chemistry with parts of the fuel system and gumming up.
Old-school diesels are really very tolerant of varying fuel blends as long as a) the viscosity is about what the injection pump expects, b) there are no particulate contaminants (filtration is everything) and c) you don't leave something that isn't an aliphatic hydrocarbon sitting in the lines and pumps for months or years. Oh yeah- and avoid excessive amounts of cyclohexane...
There's some finer print involving avoiding certain chemistry combinations and entrained water, but that's the basics. Things get pickier if you start worrying about low temperature starting (viscosity and RVP start to matter more), or non-hydrocarbon fuel components (avoid chlorohydrocarbons).
