Rotella T6 is good all year round. Though there is a bit of $$ savings with the T4. I'd adjust the valves and clean and change filters as suggested. Lots of good suggestions. I will add a diesel purge. I am blanking on the German brand of injector cleaner to use. You feed it uncut into the fuel line. Lots of smoke.

Also, if slow, check that the bushings are in the 'throttle' linkage setup. Also, that the pivot ball is tight.