Engine oils.

You might want to check out Tractor Supply's "Traveller" 20W-50 ($17 for a 6-pack of one-quart containers, which works out to $2.83 per quart) and 15W-40 diesel-rated oil ($51 for 5 gallons, which works out to $2.55 per quart).

Best bang for the buck, IMHO. I think the diesel-rated oil contains ZDDP, but I'm not sure about the 20W-50.

You have to order in the 20W-50 oil ahead of time because they don't normally stock it.