My 1989 4WD with a 4.2lt diesel engine can be tuned to emit no smoke at all (even on full throttle with a heavy load). After some time, the fuel mix will get richer and it starts to smoke a little bit. If I avoid adjusting it I can produce thick black clouds with every gear change (I actually like a bit of black smoke, it means the engine is running rich. Not very fuel efficient however the extra fuel in the cylinders does provide lubrication and helps cool the engine which is important in Australia when climbing steep hills in the summer).
Just because modern tractors (as well as trucks and cars) don't tend to produce black smoke, it doesn't mean they can't.
But if you just over spent $100,000 on a tool that is vital to maintaining your income and lively hood, do you think you'd pinch pennies by not servicing it?
Hell, if it was close to it's due service and about to be used hard, I'd send it in early to make sure there would be no problems during the planting / harvesting etc.
If it were possible (and it IS, Giants please take note here and stop making the modding community do all the hard work for you), the vehicle maintenance mechanics should based on several factors rather then just engine hours.
Eg, Engine hours are used to handle servicing requirements, ie oil change / tune up. Exceed the recommended hours and you start getting blacker smoke, slightly less power etc.
Engine hours multiplied by workload (eg, 100hp engine pulling a 50hp implment would by x1.5, pulling a 100hp implement would be x2) with additional multiplication based on hours overdue for service would affect engine wear (the time before the engine starts having expensive break downs and eventual engine failure).
Using smaller implements with regular servicing would mean a longer life of your tractors engine.
Other factors could also be included, such as tyre wear (with options for tyre changes to include taking it to the store, or buying tyres on pallets and "changing" them at the farm), suspension / chassis wear & tear (driving fast over ploughed fields wears them out faster) and overall damage (such as crashing into trees) could all be implemented to have various effects including reduced functionality and reduced sale price.
That however is far beyond the scope of the Seasons mod. But if they wish to add a small cosmetic feature like black smoke, then they have my full support.