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Originally Posted by sasilverbullet
jettech - how does a thinner oil disappate heat better? and, if the flash point is higher for the heavier weight oil, then how is that not better?
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A thinner oil will carry the heat away from the engines hot spots at a faster rate, it will also dissipate that heat quicker instead of retaining it. A thicker oil will tend to retain the heat and cause lots of parasitic drag, of which will actually cost you horsepower, usually in the neighborhood of about 5 to 8hp (50W to a 30W).
Flashpoints are basically just a number, sure it looks good on the spec sheet if it's really high, and it "should" keep the volatility very low, but after the oil has been used for a while most come in around low 400's, even a lot of dino oils come in that high. I don't want to add a lot of confusion here because your intentions and thoughts, from what you've read about the 15W-50, are correct. BUT, in real world testing a thinner oil is almost always a better option than a thicker oil for lower bearing wear and ring wear.
Now that I've given you the technical aspects of the thinner vs. thicker, (and believe me there are many more) it would be best to send a sample out for an oil analysis to see if this engine can handle an oil this thick. It "might" come back great and you'll feel better knowing you can use it from now on and not have to worry about excessive wear rates.
I hope this helps,
Dale