I've changed about 5 Volvos and my MB 300E to synthetic oil with no leaks appearing. All had at least 100K mil;es and one Volvo had over 200K miles. The 300E had 140K miles. I've used both Amsoil and Mobil 1. Mobil 1 is a bit less expensive.

I change out the synthetic at about 6000 mile intervals. I used to change my regular oil at 3000 mile intervals. There is a web site which has published lab analyses of both Amsoil and M 1 over about a 15000 mile change interval and these test show there is no loss of lubrication in that interval. I don't like to go so long so I use the 6000 mile change.

Results:
1. With regular oil, I usually found that the oil got quite thick by the 300 mile change time, very black. The synthetic oil does not thicken and seems less dark at the 6000 mile change.

2. When using regular oil (usually Havoline) I usually needed a quart of replacement oil sometime in the 3000 miles. I rarely need to add synthetic oil in the 6000 mile interval. My theory is that the regular oil's lighter molcules are burning off or else the oil is breaking down to lighter molecules.

3. I can't really report any improvement in gas mileage with synthetic oil.

I think you can do fine with regular oil using 3000 mile change intervals. However, if you live where the winters are cold, synthetic oil will floww much better and definitely helps the engine. I use synthetic oil partly because I like my cars and want to do the best by them.

Many recommend synthetic oil in the tranny and the diferential.

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1986 300E
1993 Volvo 940 Wagon
1984 Volvo 240 (daughter's)
1976 Dodge 1/2T pickup, gas hog