Whats the best way to get stubborn tree-sap off of my truck without damaging the clear-coat?

Clay bar is the only thing that will not hurt the finish and remove the tree sap. The problem with using abrasive compounds is that they will hurt your clear coat, same for sap and tar remover, sap and tar contain abrasive dirt, that when you rub to remove them will scratch your finish.

there are many products on market for removing tar, film and inclusion sin paint work. look for one called 'clay bar'. this is very good at removing small hard surface contaminants and after polishing can leave a car like new ,though can be a bit hard work as its time consuming so one panel a day is best.

If its dried and hard to your automobile the outside might already be damaged... Mainly if in scorching weather. In case your car is a late model are trying kerosene or in case your cautious laquer thinner will melt it correct off.... But it will go away a dull mark which which you could without problems hand polish off with a smooth cloth and a mild chopping compound. If its particularly bad you might need to clear the majority of it off with a razor blade... No kidding! Put covering tape around the sharp corners of the blade and slice by means of the sap with the blade lying close to flat on the car.

get an electric buffer and buff your vehicle. start with a coarser compound and work up to the finer ones.

dont sit in one spot too long or you can burn right through the paint.

or pay someone to do it. that bug/tar remover stuff works for stuff that hasnt been on the paint too long but if its had any time to harden (via sunlight, engine heat, etc) that stuff wont work at all and you gotta buff it out.

Bug and Tar remover.

http://www.turtlewax.com/main.taf?p=2,1,4,11

turtle or dupont bug/tar remover..ive used kero and gas to clean mine,,,Do NOTuse a green scrubby pad,[don't ask lol]