Engine oil change question?

My car manual says to change my engine oil and filter every 7.5 months or 7,500 miles (it does not specify if conventional or synthetic is to be used). I do not want to use synthetic oil because of future leaks may develop. So my question is.. can CONVENTIONAL engine oil last 7.5 months (7,500 miles)? The same question goes for the oil filter. I heard you need to change your oil every 3,000 miles so that is why I am asking this question. Thanks.