203s all use 10W40. I have an NP205 in my 88 crewcab dually, and it uses ATF!!! I thought about putting gear oil in there instead, but that's what the factory put in, and they have just a tad bit more engineering experience than me, so I left the ATF in it. 
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ken
'58 Chev NAPCO 4x4 short
'60 GMC Suburban 4x4, restored
'60 GMC 3/4T stepside 4x4
'64 GMC Suburban 4x4, restored
'90 Chev Suburban 4x4, dually, ZZ502
'77 GMC Suburban 4x4, one ton axles, 12" lift, 39.5"s
'88 Chevy crewcab dually 4x4, 10" lift, 42"s (x6!)
'03 Jeep Wrangler Rubicon
'79 Chev 3/4 Suburban 4x4, 400-400-203
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