Keep something that has a useful alternate use. Motor oil (for your engine, not someone else's), diesel, B100, food-grade veggie oil, ATF, etc.
If you're keeping it in your trunk, maybe a quart of each. Sure was glad to have the spare quart of oil in the trunk when I ran the Horizon out of oil on some back country roads a number of years back. Pulled over, put it in, drove gently to the nearest filling station (~5mi) where I filled it up. It never ran quite the same after that, but it did go another 30kmi and a number of cross country trips before the frame rusted out from underneath it. So yeah, I can understand your desire to have some spare whatever in the trunk
