Synthetic is the single best thing you can do for your Rex engine IMHO. You will get the extra cost back and more because the engine will last twice as long before needing that $3500 re-build.
Hauling heavy loads + lots of city driving + hot-running turbo engine
puts you in the *severe* service category, no ifs ands or buts. Whatever synth you go with, I'd change oil and filter every 3750 just like the manual says.