Whats the best gearbox oil to use for a 1996 RM 125?

According to the manual (Suzuki RM125), it says to run 10W40. They make an 80W gear oil and I have used it but it did no better or worse than automotive 10W40. 80W gear oil is approximately equivelant to 25W engine oil, because engine oils and gear oils use a different rating standard