[AT] OT. Grasshopper mower

[H. L. Staples]

Last week I purchased an older model 616 Grasshopper mower.

After replacing belts, blades,fans and tires the unit seems to do a good job
and should work well for me. 

Thought it might be a good idea to change the oil in the hydrostatic drive
units, dealer gave me a price of nearly $90.00 per gallon for genuine
Grasshopper synthetic drive unit oil.

Any one know what would be a safe substitute oil to use ???.  
 
H. L. Staples
McLoud, Oklahoma
USA