Guys, please lets not follow the "other local Aviation COMunity" website
We need to encourage questions, if it has been dealt with previously, lets follow Andy G's response - asked and answered - no one is forcing us to read this, move on if it frustrates you.
For what it is worth - you will not recoup your costs by using synthetic - in my opinion it is superior to mineral oil, but the only savings is probably going to be frequency of spark plug changes and decokes possibly.
Any "decent" oil will get you to the mandatory inspection time. I have never used synthetic oils, but my first 503 got me to 750 hours with original parts - using maily chain saw oil. Hangar mate only used synthetic oil and he sold his plane at 550 hours and still going strong.
For me, more important than the oil one uses, is how the engine is looked after, specifically when it is not used frequently. An instructor from the Cato area had an engine out on a 503 with less 50 hour since rebuild, but the rebuild had been done some 3 years earlier, the problem was found to be a failure of the big end due to damage caused from corrosion.
So either regularly fly, or at least run the engine up to operating temperature, or refer to the manual as to how to store the engine for some time so as to avaoid damage due to corrosion / moisture ingress
Have a look at this site for additional information -
http://www.theultralightplace.com/choosing_oil.htm