I think you were treated very poorly and deserve better from the facility that you paid to maintain your aircraft. Amongst other things I have done, I ran a Cessna Service Centre for some years (as an integral part of our business).
We always went to great lengths to ensure that we always understood and had the approval of the customer for anything we undertook to do. This was always done in writing, with estimated costs, and required additional (written)approval before any change to the basic workscope was initiated. The customer was always invited to come and look at anything that we found, so they had always knew what was happening and why. At the conclusion of all maintenance activities, all replaced parts were returned to the customer.
In the event of a failure of the type you describe above, it sounds as if the maintnenance facility has erred in the way they handled the problem, but wants to put the responsibility back on you. If the cooler failed under warranty, then there are entitled to expect the facility would have initiated the warranty process to procure a replacement, plus to reclaim the labour cost of a warranty replacement.
Dependent upon the type and cause of failure, you are equally correct to assume that the manufacturer (and others) would have an interest in what occurred to cause it to fail. Oil cooler failures are not particularly common. To actually dispose of a failed part is pretty strange, and tends to make me a little suspicious of the events that led to it's failure (at your expense). At the very least, this failure should have been reported to the manufacturer for review. Of course, if the warrnaty process had been completed, it is automatically going to be reviewed by the manufacturer!
For any business to "eat" the costs and tell you to go away, is not a very sound business pratice. We always figure that one disatisfied customer could destroy the goodwill that 100 satisfied customers could generate. We worked very hard to ensure that our customers were satisfied with our work and standards, and would always make a point of asking them and following up to make sure that it was. Not to say it always worked. We had our share of problems over the years, we always worked to remedy them, but it often left a bad taste in the mouths of both sides. I certainly had customers that I had resolved issues with in as courteous and satisfactory manner as possible, who were distinctly unwelcome in our facilities, and we would always be too busy to work on their equipment (the worst were some government agencies!).
I would suggest that you contact the owner of the facility directly and address your issues. They are pertinent and fair. Any self respecting business owner would discuss this issue, even if for some reason he disagrees with your position. There is a pretty good chance that he is either unaware of the issue, or the full facts of the matter. When you commit to pay for a service, you are entitled to expect full and fair treatment from the service provider.
As a footnote to all this.
We got out of the entire business of 3rd party aircraft maintenance, as it was simply not a viable business proposition for us. Dealing with Cessna was a nightmare, full of officious idiots and the whole company is run by "committees"!
The margins are so slim, and the risks so great, that we just decided to concentrate on our core business. It is the best thing we ever did. Trying to deal with owners of aircraft older than any viable vehicle on the road, who simply couldn't understand the corrosion, electrical or other system problems that occurred as a part of the age, condition or maintenance standards of their machine was extremely frustrating (its not my fault). I don't set the price on parts, nor the time to perform repairs, but we invariably had to eat all kinds of costs simply involved with the age and condition of the machine. Some of the requests and ideas for "cheap" repairs would blow you away - and that action would carry our name and liability!
Nah, I sleep easy now, and often wonder where that guy is, who told me that his typical annual cost him $25.00, is today?
