My 57 6400 uses plain 30 wt. Non-detergent is best. Doesn't foam. Be sure to lay a 4x4 post in the hinge crotch if any service is to be done with the lift up. Safetysafetysafety. Should a hose burst that bed can slam down before one can get out of the way.

~~ Jethro
1954 3100
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In the Dity Gallery
1951 3100 (gone) / 1956 4400 (still in the neighborhood) / 1957 6400 with dump body (retired) / 1959 3100 panel (in the woods junked) / 1978 Custom Deluxe K10 / 1993 S-10 4.3 / 2004 Chevy Crew Cab / 1945 John Deere H / 1952 John Deere B / 1966 John Deere 2510 / 1967 John Deere 1020