I don't think whale oil was used, it was too expensive. Some was used as aerospace and limited slip differential lube back then but the whales were near extinction and only some critical uses would need the oil. Definitely nobody without a serious dislike for their money would use it in a lamp!
I remember draining the original fork oil from those bikes, it smelled more like fish oil. Just use fork oil. With SAE10 you should be good, if they are too stiff mix 5 and 10 about 50/50. Same if not enough damping, mix in some 15 or 20.
High mileage forks may be a bit worn and need thicker oil to get good damping.