e anderson -
I may be wrong, but it is my understanding that the only 'tighter clearances' in a motor calling for 5W-20, is in the gear tip dia. / pump body, of the oil pump, to ensure adequet oil pressure w/thinner oils.
The clearances in the oil pumps used in motors calling for 5W-20 are in the neighborhood of .0020 - .0049", whereas the oil pumps in motors calling for 5W-30 or greater are running around .0078".
Any manuals I have seen indicate no clearance or nominal tolerance changes to journal bearings, piston bores, etc. over the past 15 years or so.
Hence, you are probably OK to run 5W-30 in a motor calling for 5W-20, but may sacrifice a little oil pressure running 5W-20 where 5W-30 is called for. (IMHO)
[ February 22, 2006, 12:58 AM: Message edited by: Dr Cpk ]