I have to agree on wrench quality, you get what you pay for. Then again, I have Craftsman :)

If you ever had used a quick change lube place, it may have been over torqued. A good pair of vice grips and taps with a small hammer may be warranted.

If you are laying under the car and front bumper, lefty is still loosey. Which means that the wrench will go from the driver's side, down to the passenger side. My own spacial geometry is whacked out most of the time :)

Klaus

I don't have an S60 here to check for you (my daughter has it) so I can't confirm the correct size, but assuming that 17mm is the right size, what kind of socket and wrench did you try? What I mean is that:
1) a 12-point socket is always a disaster waiting to happen -- very apt to round a bolt-head or a nut if it isn't a perfect fit. Instead, six-point sockets are the only ones you should use in a critical situation.
2) again, some wrenches aren't as reliable as others: either they weren't made to perfect specs, or they're actually weak and flex slightly, letting the wrench slip just enough to, like a 12-pt socket, slip and round the high spots of the nut. I always try to use a closed-end 6-point wrench as my first choice -- if I can't use a closed end, a flare-nut wrench is my second choice (it grips more of the nut than a mere crescent). And for critical applications such as brake lines, I only use a few certain-sized (because they're expensive) Snap-On (brand) wrenches -- they're made more precisely and strong enough to prevent bending and slippage.

Bottom line: 17mm might be the right size, but your sockets and wrenches may be the cause of your problem.

Good luck.