My '90 3.0 has always taken 5.5 qts for it to register on the dip stick. In '90 I originally thought that I may have had oil sitting in the head, causing it to read low, so after putting in the 5 qt.s I let it warm up/took it for a short drive, and it still read low. Since then I have just added the extra 1/2 qt. Here is a post fro Marlin on the differing capacities people have come up with.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed