As an Amsoil dealer I have made some very biased observations:

1) Synthetic oil doesn't cause leaks...as stated IF you already have a leak, synthetic oil will leak faster because if it's EXCELLENT flow properties. That's how the whole synthetic oil and leak thing started. Person has very small petroleum oil leak - put synthetic in, sudddendly notices leak. Best to start synthetic oil early.

2) Good synthetic oil, such as Amsoil is actually MUCH better than petroleum oil for seal life. Many multigrade petroleum oils contain pour point agents (solvent) to lower the pour point just to get the wider vis range...these solvents are hard on seals.

3) Petroleum oils break down at turbo temps into abrasive carbon compounds (really happens after solvents mentioned above boil off). This crap chews at the seal surfaces.

All in all good synthetic oil is better for seals than petroleum oil....and I'll even take my bias hat off for that statement.

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