Since the viscosity of the oil smooths/slows the lift of the needle due to vacuum depression, the thinner the oil the quicker response, but the thinner the oil, the easier it slips past the o-ring. I use a 20w50 + Marvel mystery oil mix - about 50/50, but I have heard of others using ATF, 10 wt machine oil, etc.
TRF/VB/Moss sell damper oil, but I have never tried it - it may be the bees knees as far as I know, but my home made mixture works for me.