I didn’t know that you could buy the “grated” version of Fels Naphtha anymore. Back in 1972 when I bought my vert first washing machine, a used Maytag Model J wringer I used Fels Naphtha powder and it was wonderful!
Since it’s a soap, not a detergent it left the laundry very soft and with a nice scent. When the weather was nice I hung the laundry out to dry and it was never stiff even though I didn’t use any fabric softener or rinse conditioner. I never had a problem with soap scum, I think the naphtha somehow kept soap scum from forming.
If you’re going to use it in an FL I’d recommend starting out with only 1/4 cup, in a TL start with 3/4 cup for a very large load you may find you need to use 1 cup.
Eddie