I know it may sound like a dumb q but is it safe to put multi weight oil in a 239 flathead v8? I talked to my Grandpa who used to have these motors in his Ford and Mercs in the 40's and 50's and he said to put 20 weight in the winter but even that seems thick. He also said that the owners manual said you could cut your oil with Kerosene in really cold weather.
I can use straight 40 weight in the summer time but in the winter it gets cold here (I'm in North Idaho) and that straight stuff is WAY to thick. It's like trying to suck molasses through a drinking straw.
Also, is there any way to convert the filter to the spin on type?
Grandpa is right - that's what was available back then.
Today you can use multi-vis oils like in any other engine. I'm in northern Montana and I use 10W30 and 10W40 in my '48. It's my daily driver - year round. If your flathead is tight enough it'll even take synthetics......BUT,
If your engine has been run for years and you've started it up without taking it apart to clean it, be careful about using detergent oils. The detergents can break loose globs of goo that have hardened over the years. They head to the oil pan and clog the oil pickup and presto - you need new bearings and the crank turned.