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Well, sometimes. Some types of seals are not really compatible with synthetic oil. I think this is the case more with modern engines with newer seal materials that weren't prevalent in the older cars. In particular, I'm also involved with Cadillac forums, and the Northstar engine will tend to leak more with synthetic. People have reported switching to synthetic, having a leak, and then switching back to conventional and the leak going away, just from the type of oil used (all the same weight, etc.).
A GM engineer who frequents the board said the seals were not validated for use with synthetic oil because synthetic is "not needed, nor recommended." Synthetic provides advantages such as increased stability at oil temperatures above 300*F, but the Northstar engine never sees those temps, so synthetics were intentionally not considered as part of the validation process. The engine went through rigorous testing procedures using conventional 10W-30 oil. I consider anything used during durability and extreme environment validation to be more than enough for my daily driving activities. And I drive the car like I stole it, so I know it works.
Of course, if I were racing, or planned to change my oil over 10,000 miles, I would probably use synthetic.
Last edited by jadcock : Apr 25th, 2003 at 01:24 PM.
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