Have a mid 90s Yale fork we did some cylinder repairs and need to put oil in. I will check book spec but curious what you guys reach for? I recall my old employer and a huge construction co ran they Delvac 15w-40 for hydros and engine as they got a right deal buying a truckload at a time. Seems I can get engine oil for about the same price. Then there is also the ISO 32,42,68, etc. The fork is a 9K and we don't use daily or push it hard. Just moving equipment around at times.
I'd probably just run AW32 (10W) hyd. fluid. A lot of times you can buy it for less than motor oil.