I know it was already mentioned but I also run Amsoil fully synth. and its a 25K or 1 year interval now (with their filter), I had mine tested about 1/2 through the year which was at the 5500K mile mark and the blackstone lab results came back great with a TBN at 5.1 (active additive) 1.0 is low, told me to check back in another 3K, my year is up in the spring in which I will have upwards of 11-13K miles, I will take a sample on my year end oil change and then pending results, most likely just stick to the 1 year intervals…