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Its not uncommon for a leak to appear (or an existing leak get worse) after changing to synthetic oil. There have been many previous posts on this subject. Regular (non-synthetic) oil tends to deposit a certain amount of gummy "crud" inside the engine and if one of your seals is already worn, this stuff can help plug up a potential leak (at least that is the theory). If your seals are all good, synthetic oil will not cause them to leak.
The first thing you will need to do is find the source of the leak. Could be the drain plug, like NJ960 says. Could also be the filter is not tightened properly. Other common leak points on these cars are the distributor seal (if you have a camshaft mounted distributor), front camshaft, crankshaft and intermediate shaft seals, or the rear seal. The rear seal is going to be the most difficult to replace due to the fact that you have to R&R the transmission to get at it. Before you do anything though, it would be worth your time doing a thorough check and cleaning of the flametrap, oil separator and associated hoses and nipples. If this system is not providing enough vacuum it can cause a seal to leak, or cause a poor seal to leak much worse than it would otherwise.
As to how much the rear seal can leak, could be anything from 1 drop per week to 1 quart per week. Depends on how bad the seal is.
By the way, I changed to Mobil 1 synthetic in my 940 a couple of months ago. Before I did it the car had a very minor leak from the front seals. After I changed to synthetic it has continued to leak at about the same rate (less than 1 qt in 3k miles) but it has certainly not gotten worse.
Good luck.
Kev
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Current: '95 945, 95k miles; '90 BMW 325i, 90k miles. Former Volvos, gone but not forgotten '85 245, 250k miles; '91 744T, 150k miles
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