jugs at Walmart are 5W20 or 5W30
My question is, is 5W too light for the Model A? What say you, oh Ford gurus????
It's NOT 5W.
The 20 part is too light really, but the 5W-30 is as thick as straight SAE 30 when hot (oil THINS when it gets hot). As they cool, the 5W-30 thickens a LOT less than the SAE30 will. Really thick oil is bad for an engine - it doesn't flow well enough.
Most people don't understand the numbers, though it is all explained here:
http://ferrarichat.com/forum/faq.php?faq=haas_articles
A Quote from the article:
"I will compare the same weigh or grade of oils showing that the operating viscosities are the same whereas the startup viscosities vary:
Mineral oil:
Oil type...Thickness at 75 F... at 212 F...at 302 F
Straight 30..........250....................10..........3
10W-30...............100....................10........ ..3
0W-30..........There are none in this range......
Synthetic oil:
Oil type...Thickness at 75 F... at 212 F...at 302 F
Straight 30...........100...................10...........3
10W-30.................75...................10........ ...3
0W-30...................40...................10...... .....3"
So SAE 30 (as originally recommended by Ford) runs a cSt viscosity vating of "10" at operating temps - that's the viscosity you want ALL the time. The multi-vis 0W-30 runs closest to that all the time, and gives the best protection. the SAE 30 gets VERY thich when cold, where the 0W-30 thickens less, but is still THICKER when cold than hot. SAE 30 flows very poorly cold, the 0W-30 flows much better.
You may want to compromise for you pocket, and 5W-30 is a good compromise - still flows far better when cold.