s310t, new sand

[Anonymous [GDPR] European in the UK]

Now you should follow the manufacturers advice or risk any warranty issues, Blah blah blah. Without any information on your system in your signature advice is tricky, some suppliers do say gravel over the laterals as it does help to diffuse the backwash but from my experience most filters backwash isn't all that good leaving plenty of dirt remaining behind in areas where the water doesn't reach. The Hayward S310t seems to be just sand and not gravel.

Sand is in use in lots of filters around the world but I am not using it any more as there are better alternatives, yest it's really cheap but it can be bettered but there isn't much point in fitting anything else unless you have the correct flow for the size of your filter and that is probably slower than what you have now as slower filtration is better filtration. The pool industry generally feels that faster is better but that's up to them and of course up to you.