Having owned four older gs´s and having put over 100K miles on the lot I highly recommend using only castrol 4t motorcyle oil 20/50. I experimented with synthetics and all caused valve train noises. Most older gs (pre-84) had roller bearing cranks and ran with only 2 or 3 pounds of oil pressure and for some reason synthetic oil just doesn´t get to the heads well enough. changing castrol at 2000 mile increments with get the most out of your old gs. One note, some model gs suc as the 650 g had plain bearing cranks and I think the gsx 750 and 1100 also had plain bearings. one word of caution, castrol gtx was reformulated several years ago and is no longer suitable for motorcycles. so use only the 4t.