I just changed the oil in my 2005 S60 T2.5 for the first time (just bought it last week).
I bought the Mahle filter from IGN and it came with new washers for the oil drain plug. When I removed the oil plug, the old washer did not come off. Tried to pry it out carefully, but couldn't get it out. So, I just left it there and put the plug back in.

Next time I change the oil, any suggestions for getting the old washer out? How important is this? I checked the plug for leaks after changing the oil, and it seemed OK.

 

As long as it doesn't leak, don't worry about it.
I reuse my washers all the time.

 

The old one was compressed to much thats why it was stuck on there. It is only an aluminum washer you can cut it off with a pair of wire cutters.

 

Since it is compressed into the drain threads, it is really difficult to tell where the washer ends and the actual threads start. Also difficult to get a set of wire cutters in there. I tried to pry it off with a really small flathead screwdriver, but couldn't get it.

 

You will have to pry a little harder.