The manual for my Agility 125 calls for a SAE 90 weight gear oil, but at the local auto parts stores all the gear oils were multi-viscosity. I went with the Castrol Hypoy C SAE 80w90 gear oil as that seemed closest to a straight 90, and Castrol generally makes good products. Any issue with using 80w90? If so what alternatives are available and where do you get them?