hello!
According to teh maintenance information I ahve available for your Subaru, the manufacturer recommends oil changes under normal use at 7500 mile intervals (not 3500 miles). This information will be found in the back of your owners manual, under the maintenance schedule section. Using 7500 miles as an oil change interval, you should have perfromed 6 oil changes by this mileage; you have stated that you ahve had oil changed 5 times to this point... this is very close to factory recommendations.
I would suggest that you may want to speak with the dealership's service manager or ask for the number of their factory zone rep to make an effort to get this covered under factory warranty. Consulting with your attorney may be an option also.
It is not normal for Subaru engines to fail at such a low mileage; usually these engines will run for 200,000 miles without any major problems other than possibly a head gasket replacement.
I hope this is helpful; if this answers your question then please click the ACCEPT button to close the question. Thanks!