I think you're holding your pride up way too high. You're not the only person they have who's been loyal for many years. You CANNOT prove it was their fault for the oil leakage, simple as that. If you could then I'm sure they would pay the full bill. But seriously man there's a reason you and your family went there for 20 years and I'm assuming it's great prices. If you have a good relationship with your SA then just stay. $70.30? I know it comes down to principle but this comes to down to pure stupidity.
My .02 cents.