"Is 5 w 30 to thin for a 36 flathead?"
No, it's not too thin. Why would you think it's thin?
Oh ... the 5W.
After all that's been written about oil, it is amazing how few understand the numbers.
First the "30". That is a viscosity GRADE, not a weight. So 5W30 has a grade of ... 30. So-called "straight weight" 30 also has a grade of ... 30 ... and is labeled SAE30. Both have a viscosity flow approximately the same at OPERATING temperature.
0W30, 5W30, 10W30 and 20W30 all have a viscosity grade of 30 at operating temperature.
What's 5W? The W means WINTER. It does not mean "thin". It does mean that when it's cold, your oil will not flow like molasses but will try to flow closer to 30 grade, bur not thinner. It's not going to be some weak, watery stuff.
At cold (cool) start up, even in the South, SAE 30 ("straight weight") is thicker than it is at operating temperature. Most wear occurs at start up ... why would you use that crap when a multi-grade oil (note it's not called "multi-weight") will lubricate your engine better?
5W, 10W, etc. all indicate the oil's ability to flow closer to the viscosity grade when the temperature is below operating level.
If you want a 30 grade oil, pick the Winter rating you want. Keep in mind that the stuff they put into the oil to do this viscosity magic can break down with extended or extreme use like racing ... but for our engines, old or new, multi-grade is the only wise thing to use.
Note:
"In my opinion, the answer is 'Yes' - I think even a straight 30W would be better than a 5W. BUT - I have not attempted to try it, so this is just a gut feel."
(Same thing happens to me. Stay away from broccoli.
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