I carry extra fluids for the vehicle in my tool box. Oil, coolant, brake fluid, wd40, power steering. As well as some recovery gear like jumper cables, tow strap, spare tire, d rings, a come along and assorted tools.
Personal stuff, I rock a 2g igloo water cooler at work and there's usually a little left at the end of the day. If not I refill with water and ice before I leave work so I always have water in my car. I also always keep a few sets of extra clothes in my car, at least 3 full pairs . I always change clothes before driving home from work. I don't like to leave my dirty clothes on and I don't like to drive with my work boots on. Also some days I forget were going on the boat instead so always nice to have clothes on the standby.
I always keep snacks in my work pack. Crackers, can vienna, ramen noodles, boiled egg, leftover lunch, something so I got food.
Essentially with the kinda work I do my gear can easily be swapped/used.