zinc oil useage?

What do you mean by "oil is changed after each session"?
decades ago an experienced racer told me that he changed his oil (& filter iirc) after each Saturday night session & at the time I had just accepted it as gospel thinking how toxic that a dirt track environment must be. it certainly does sound excessive & later someone else has said they changed the oil every time but only the filter after several races. I am open to thoughts/suggestions. RR