The fit is not just "dry" ; it is what is called an "interference" fit. That means it the fittings deliberatley make tight contact...too tight to go all the way together. This is so when the solvent melts the plastic, there is plenty of material to form the weld joint.
No amount of lubrication would let them go together. But you can judge the depth of the socket, and when the joint is prepped and solvent applied, it should go together to the full depth of the socket when pressed in place. You can build a little fudge factor into this.