Unless as mentioned above, if youre in harshly cold temps or a track monster (neither apply to me) you'll be fine eyeballing. Im OCD and had to use oem g13 only. I was lucky my dealer had it on hand for even cheaper than what I could get it for online surprisingly. And if you're a tad low, topping with distilled water is fine. At one point, the light came on and I was alarmingly low, reservoir turned out to be almost empty! I was sure I had a leak somewhere, but no evidence in and around the system. refilled with 50/50, after a few drive cycles, nothing. *shrugs* Weird.