The level is really not critical unless you really overdo it. I top them off until you can see oil in the piston rod . What is of importance is the oil you use. It should not be heavier than a medium weight engine oil, say 10w/40, and in the past lighter oil was used than that, rather like light machine oil. Certainly, if you went significantly heavier than 20/50 you would end up with some very untoward effects - usually overrich mixture. If the trouble only started after you topped the dashpot, I should try running it without the piston damper in to see if that makes any difference