always use the suppliers provided values.
you don't need a stretch guage ,just use a digital caliper .
39 ft/lbs is plenty for a 5/16 bolt.
---here is what you are looking for ,lets stick to rods as a example .as the crank is spinning around and the rods are being accelerated forward and backwards what happens is the 2 halfs want to seperate ,ie end cap from the rest of the rod.so the bolts holds them together.if there is not enough tension from the bolts to hold the 2 pieces together the halfs will seperate and come back together.this slapping action will break bolts quickly.so a proper amount of preload on the bolts is used that will applie enough tension to keep the parts firmly together to stop seperation .as you can see it is equally important to not over torque/stretch a bolt as it will fatigue due to being over stretched and loose all its ability to applie tension.
so lubing the threads and bolt heads will simply cut down on friction when torquing the bolts so you will get a better and more accurate reading.no loctite is needed as the bolts are always under tension and will not slack off.
