The argument against using synthetic is that his is an old car (88), so probably it has not used synthetic oil till now. On this forum itself, there are a lot of threads that suggest that you may face gasket leaks if you switch to synthetic on an old car that has never used synthetic before. So why spend the extra $$?
Also, if it is mostly highway driving, then one can safely increase the oil change interval to at least 5000 miles. The oil deteriorates much more with cold starts and stop and go driving. 1500 miles a week has got to be mainly freeway runs.