My chandler had some soft plastic tubing that was rated for Diesel. It has a slight yellow colour when new, and now after 6 or 7 years it hasn't changed colour at all, remains perfectly flexible and looking new.
I don't use it on my boat. Its on my heating oil tank at the cottage. I would be uneasy with sight tubes on the boat, in the ER where my tanks are. I use a dip stick if I don't trust just reading the hours since the last fill.