These seats are designed to be newborn-friendly. Any of them causing any physical damage to a child is unthinkable. Such seat should never have been made and accepted for sale. Unless you bought the cheapest seat on a flea market, you're going to be ok.
Long trips are discouraged because your child is restricted to the same position for their duration. If a baby is placed in an unnatural, non-physiological position, it is going to stay in that position for a certain time. You should make sure the baby is placed in the seat comfortably, that it's back and head are supported, that there are no unnaturally bent legs or hands. If something is wrong, your baby is most likely going to let you know - through crying, of course.
If you're really worried, make 5 minute stops every half an hour.
1,5h is not that long time. I've friends who made 4h trips on a weekly basis which started when their son was about (less I think) one month old. Their LO is 2 years now and has no seat-related health problems.