I rarely carry cash, but when I've received assistance with windscreen wash, oil checks, tire pressure, etc, that is when I look for cash to say thank you.
Yes I do tip them. It also depends if they do the extra stuff that they suppose to do. I can talk from experience because I use to be a petrol attenant back in the days and they earn very little money. Sometimes your tips are more than your actual pay
At my local garage, they wash all the windows even if I only put R200 in. I tip them R10, same as car guards at the mall, so long as they push the trolley and pack away the grocers. They are usually able to break a 50 if I haven't got change.
I very rarely have cash on me (come now, it's 2020). So until they build in a decent 'would you like to add a tip' type thing to their point of sales units, I unfortunately don't