Took my car back to the garage after a renewed small oil leak offered visible evidence to invoke the guarantee. This time the owner was behind the counter, a tremendously experienced fellow who said:
"Yeah with Subarus if one cam seal fails, they all need replacing. Oil squirts out. They're famous for that. And you have to use a special sealant too, or it won't seat right. If I'd been here I'd have said to replace all the seals, not just that one."
Based on some old posts on this site I took this opportunity also to replace the water pump, in the name of preventive maintenance. The tally was over $400, from which he deducted last week's $188 bill--so for about $250 I now have a purring, happy engine which will be good (one hopes) for the next 88,000 miles.