Graingers has a test kit available. I believe you ship it to Parker for analysis. It's not cheap. OR, switch to Exmark's Hydro oil. I've never seen an Exmark or Toro that uses Mobil 1 that didn't darken or thicken. Turn black? No. It's not as great in hydro's as everyone thinks. One reason it was used in the first place was the hope it would hold up to the excessive heat that system creates due to the small capacity. On hot days, the oil in the resovoir can exceed 200 degrees.The new fluid is far superior to Mobil 1. Kit number is 4va89. Research it from there. So before this post gets torn apart by the "experts", try it and judge for yourself. You can flush that entire system with a little creative plumbing.