[Johnc]

That's a very good question. You probably can't use color because of the mixing of the new with the old. Transynd is noted for its extreme heat tolerance (330F intermittent), so perhaps a frying pan heat test? Take a sample from your transmission and compare it to a standard oil sample as you heat the pan? I have seen heat tests with synthetic versus mineral engine oils, and the time difference in the oil starting to darken is fairly significant. Be careful whose frying pan you use