Why are late model Subaru Foresters required to use synthetic oil?

Refer to the owners manual for what oil is recommended and use that. Salesmen sometimes don't know anything about the cars they sell.

Yes, I know what oil to use; I wanted to know why and I think I found the answer. Synthetic oils do a better job claning the engine and timing chains are sensitive to dirt.

I really don't think it has all that much to do with the timing chains. The oil filter traps most of the particles going through it and all oils today do a good job of cleaning the inside of an engine provided the oil is changed as recommended or sooner. Synthetic oils are more robust and will take a higher temperature than conventional oils before turning into "goo". This is especially important in turbo charged engines (not sure if yours is a turbo or not). Run what the manufacturer recommends and you will be good to go. Best of luck with your vehicle sir.

They can extend the oil change intervals by specifying synthetic oil. New Subarus also have low tension oil control rings for fuel economy and use 0-20 oil to help with fuel economy. The downside is some cars have excessive oil consumption due to the rings and thin oil.

I talked to my dealer about this and he said that using 5-20 oil may result in poor idling especially in cold weather due to very tight engine clearances.