If you have a way to circulate the fuel, I'd add a conditioner. Two I've had good service
from are Algae-X and Archoil 6200. Since first using Archoil in my diesel pickup, I use it all the time in the boat
. Better winter
starts, the usual biocide, makes it easier for your primary filter to trap water
and seems to give me about a 10% better economy.
Diesels don't usually seize, but you can pull the injectors on most engines and put some oil
on the rings. Smokes on 1st startup.