That's a good question. If I remember right, and I rarely do these days, I think it was a compatibility issue. I might have had to flush the system because 5 was silicone or imcompatible with the 3 but there 4 would cause no problmes. I think the 5 might have had issues with absorbing moisture also. Some one here with chirp up with a correction if I'm wrong. I just bled my system and realized that the new fluid I added was synthetic and the old was not. I guess I should have read the jug better.
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