The higher the number (20, 30, 40, etc.) the thicker the oil is. It also does not flow as well or drain back down to the pan, and sticks to the cam, crank, pistons and everything else. All that means is that it creates more friction and reduces gas mileage.
The second time I rebuilt my 292, I used 20W-50 racing oil for about 7000 miles through the first 3 oil changes. At the time, I was very anal about keeping meticulous records, especially fuel consumption. That particular time, I didn't have any 20W-50, so just put in regular 30-weight. My gas mileage went from 11 to 12.2 mpg. Surprised the heck out of me, so I experimented for the next few years on all my vehicles. Same or similar results.
I now prefer to use a multi-grade 10W-30 in older engines (pre-1987). Newer ones that have port fuel injection are often built with less clearance in all the bearings, so can use 5W-30. That's what goes into my Toyota and my Dodge Neon.
Some types of old designs or worn out engines MUST use higher numbered oil, due to the large clearances in the bearings. All-out racing motors should use higher numbered oil due to the extreme stresses on the crank and rod bearings. For your stock 250 engine, use a plain 30W or 10W-30 and it should be fine. You probably don't need 40W.
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David
newbie #4153
