Never had to deal with these, but thought I would point you in the direction of bugspray.com. http://www.bugspray.com/articles98/powderpostbeetles.html

They discuss use of Timbor and recommend the use of a penetrating formula called Boracare. You might want to give that a try before coating your frame with used motor oil. You probably do not want to increase the flammability of the garage and deal with the odors, toxicity and mess of used motor oil.

the surest way to kill the beetles with a full tent fumigation using either bromo-gas or vikane its expensive but its the only 100% kill treatment as long as they put the right amount of gas and keep the tents on the right amount of time.If you get this treatment done the only thing I recommend is to try and mark all the exit holes with some kind of mark so you can see if new exit holes show up which means new activity. Good Luck.

What would a tent cost approx? Barn is about 4000 sqft.

the cost is determined by structure size,measured in cubic feet.length x width x height. i would need the average roof height usually you figure the peak height and then the eave height if peaked or the roof height if it is flat, then you measure the sides.a 4000 sq. ft. structure with a 12 ft. average roof height would be around 47000 m.c.f.here in san antonio, tx the price would be around $5800.00 dollars. the price is a little higher here because no one local does treatments.