Please Don't Judge My Question

My little girl is one week old. At her pediatrician appointment we were told that a rectal temp of 100.4 is an emergency for newborns under 2 months old. I noticed when they took her temp they used some sort of jelly or lube before inserting the thermometer. I'm not currently concerned about a fever... But I was wondering if anyone knows what they use or if Vaseline is safe cuz I have a tub of that and I know it has multiple uses... I just don't want to torture my little girl by going in dry if concern arises...