The fork oil should be changed every 4-5,000 miles.
Factory spec on the fork oil is of premium quality ATF (automatic transmission fluid), however we recommend a 10W (light rider) to 15W (heavy rider) motorcycle fork oil. ATF is approximately 5W and replacing it with a heavier weight oil will stiffen up your front end.
Recommended fork oil :
CB350 / CL350: 200cc (6.7 ozs.) Early style with external spring
CB350 / CL350: 135 - 140cc (4.6 - 4.7 ozs.) Late style with internal spring
SL350: 180~190cc (6~6.4 ozs.)
How to change the fork oil (Instructions, parts and video)