I use oem volvo seals and have no trouble with them. Acouple of suggestions- make sure the oil is not dripping down from the distributor or valve cover gasket or cam tower and running around the bellhousing to look like the rear seal. If the oil is coming out of the bellhousing, make sure the gasket on the seal housing is not damaged. Another possibility is that there is a small imperfection in the lip of the seal which will wear away in a month and the leak will quit by itself. If the leak is minor, maybe just get confirmation from your shop that they will look after the problem if it remains, then drive the car for a while. And, like about a thousand people are itching to say, clean out the crankcase breather.