Skin condition....... HELP PLEASE

At this point probably best to take him to vet. He probably has a sensitivity to certain chemicals. I would
stop the motor oil thing for sure. I have a Black and Tan Catahoula that when he is healthy has the shiniest jet black coat there is. At least once every couple years things go south in a hurry. Vet took scrapings each time and cant never find anything. I now have a routine I use when he has an issue and have good luck with it. As soon as I see a dullness to his coat are a raw spot are he starts shedding a lot I treat him with Prolate-Lintox, 1 oz. to a gallon of water. Use a wash rag or sponge and thoroughly soak his skin. Once a week for 3 weeks. In between I use a skin moisturizer/lubricant from the dollar store and apply to his stomach and inside of his legs and such (areas with not much hair). Like you NU-STOCK didn't do anything for him. When he's healthy I have to be careful what I spray on him so as not to irritate his skin. I use
Absorbine Ultra Shield for insect control on him. It has skin and coat conditioner in it and I add Skin So Soft to it. I know a lot of folks will say that's a lot of trouble but I like the dog and that's what works for him. The Vets were no help. Through trial and era this is what worked . He hasn't had an issue in over a year.