[Flathead Fever]

Because the sulfur and other anti-wear additives have been removed from most oils. I use the Delo 15W-40 "diesel" oil in all my collector cars, flatheads and Mustangs. They cannot remove the additives from the diesel oil because of the extreme pressure the bearings have on them. You need those additives for your flat tappet camshaft. I was fleet mechanic for 30-years at the phone company They kept their vehicles for over 15-years and some for over 300K miles The oil we used was Delo 15W-40. We used it in cars, we used it in heavy trucks, didn't matter what the underhold label called for it got Delo 15W-40 and we never lost and engine from wear. Hundreds and hundreds of engines. We lost a few from them running completely out of Delo 15W-40. Usually, the vehicles fell apart before the engines wore out. In 30-years I only saw one camshaft lobe go flat and that was on a Chevy and they were known for that. That is some good oil.

Non-detergent oil was/is made for engines without an oil filter so the dirt will settle to the bottom of the oil pan and not continue to circulate in the oil. It is almost impossible to find anymore, except in the garden section because small engines like lawn mowers still recommend using a non-detergent oil. Detergent oil keeps the dirt suspended in the oil so the filter can remove it. Since a lot of flatheads never had oil filters non-detergent oils was what they used. If you don't have an oil filter, just use the modern oil and change in when it starts to get a little dirty rather than waiting until it's really dirty.