Generally speaking, if you're not running on the track, you probably don't really need a cooler. On the street,
oil and water temps generally stay pretty close together. If you run a 180* thermostat on your 351W, you'll probably keep the
oil temps below 200'ish.
Without a cooler, the temps will change rather slowly. Water temp may climb to 220* in slow traffic on a hot day. But once you get moving it will drop rather quickly. The
oil temp won't respond as quickly without a cooler. During the summer I would run
synthetic oil, because it can withstand heat spikes better.