In the 80's I put all the SJ goodies on my 90/6. The anti-bottoming springs worked great to make the curb drops smooth. The nasty sticky Shell fluid was a pain. Went to ATF and worked just great.
Fast forward to my 79 RS. Same curb drop clunk with stock setup. Wilbers springs solved that pronto with no preload. Now using Bel-Ray but will go back to ATF when that runs out. Brand name MC fork oils are just fine, but at three times+ the cost, not frugal, as they simply will not last even twice as long as just ATF.
I can't image running motor oil in the forks, multi or straight grade. Makes no sence at all when ATF is better and costs no more. It certainly is much closer to spec weight than anything but perhaps 3in1 oil.