Propane in tanks will last a very long time (decades perhaps?) with no intervention.
Long term diesel fuel needs to be treated with biocide because bacteria will grow in it.
The automatic transmissions we ship to the Army as service backups often are in storage for long periods and we have to treat the ATF with biocide.
http://www.powerservice.com/bk/
thx for the tip! but the propane set up costs $$$ i don't have, but i DO have the generator and the diesel is filled every year... and the cost of a large propane tank isn't cheap and renting one costs money too but for people that heat with propane it's the hot setup