Average care is driven 15k miles a year, 4 months is 1/3 of the year so 5k miles. 5k miles is a good interval. Despite what people say 10k-20k miles on synthetic in a modern engine is a bad idea. If you spend anytime on here you will have seen tons of people talking about failed turbos or timing chains even though followed manufacturers recommend intervals. Most engines are turbo now, higher compression ratio, much higher temps, this is not comparing synthetic oil to non-synthetic, as non-synthetic is not usable in these engines. In a vehicle that can take non-synthetic? Good synthetic oil will last 20k miles. On a high demand modern engine? 5k, or be prepared to spend big $s on new turbos earlier then you would of had to or replace stretched timing chain(s) that got to hot. Synthetic oil is probably 5x better than non-synthetic, but those engines can not run non-synthetic at all cause they are way harder on oil. So oil got way better, but we made engines that wear the oil out faster.