For situations like that I would just use an over-the-counter (OTC) shampoo mixed extra strong. A good one to have around is Mother's California Gold because it's really inexpensive but a comparatively high quality soap. It says one ounce per gallon, I think. When the car is super dirty I just do about 1.5-2 oz per gallon. I was using
Pinnacle's shampoo for a while but when I run out, the only two I'll have in my inventory are Mother's CA Gold and Optimum No Rinse (ONR.)