I'll provide the unpopular opinion that the SOHC4 used on the street simply doesn't need an extra oil cooler.
If you are asking for 80 to 100 % power output from the engine for long durations and in high ambient temps then you should have an oil cooler to give your oil longevity and thereby save the engine from poor lubrication.
Look where the heat goes from the engine.
1 - the exhaust
2 - the cooling fins
3 - the oil tank and filter fins.
The SOHC4 has a very large cooling fin surface area. Oil coolers have very small surface area by any comparison. Further, they are usually mounted in front of the engine. Any heat they dissipate passes to the engine cooling fins, reducing their effectiveness.
Oil circulation in the SOHC4 only incidentally passes near the combustion chamber heat source.
All the heat removed from the engine is transferred to the surrounding air.
Air has a thermal conductivity of about 0.026
Engine oil has a thermal conductivity of 0.15
Aluminum has a thermal conductivity of 210-250 depending on alloy.
What do you want to use to carry heat away from the engine?
I don't think even the oil cooler manufacturers provide any comparative data of oil temps or engine temps before and after the cooler installation. It's for certain I've never seen published or documented findings for the SOHC4.
The proliferation of oil coolers seems to be based on the "cool" factor of looks, or the everybody-else-has-one-so-it-must-be-good reasons.
But, don't use an oil cooler to cool the engine. Use it to cool the oil so the oil will last longer if that is what you need. An oil break down will cause increased friction and higher resultant engine temperatures.
Where does the oil gain the most heat? At the engine crank and rod journals when under high load. The film of oil becomes highly compressed at this point and the compression causes the oil temp to skyrocket. It is not conducted into the oil, it is generated within the oil. If you want to monitor peak oil temps, do so right after it leaves the crank and rod journals, before the engine case has a chance to lower it's temperature.
Why do you want to know this? Because, if you know that the oil has been overly abused you might want to change it earlier than the 1500 miles where viscosity break down would occur anyway.
I don't believe an oil cooler will allow extending the oil change interval for a street engine. But, if you are driving extended distances in the desert heat at high speed, it could help the oil retain most of it's qualities up to that change interval.
If you want to find out if your current oil cooling system is effective for your needs, monitor the oil as it is being delivered to the main oil feed galley. If the temps are routinely getting over 230-260 F, then you are likely damaging your oil under high load conditions and shortening it's useful life. Either use an oil with higher temperature tollerance, change oil more frequently, or add an oil cooler.
Overcooling could be worse than undercooling. The oil's operating temp must be in a certain range to squirt though the engine clearances as the designer's intened. Too cold and the oil doesn't flow properly and starvation effects can be encountered. This is why oil coolers must be fitted with a thermal bypass valve to prevent cooling of oil not up to operating temperature. A secondary issue of the thermal bypass valve is that during cold weather, it may trap some oil in the cooler when the oil never gets to a temp that needs cooler circulation. The danger here is that the oil in the cooler never gets to a temperature that allows water condensation to vaporize. The water, in turn, forms acids in the oil and begins erroding metal bits in the engine. Changing the oil in the cooler can be problematic in cold weather months, particularly if you can't get the thermal bypass valve to open up.
Are oil coolers for SOHC4's necessary? Perhaps in special circumstances, yes. Normally no.
Are they effective? Depends on the engineering of the oil cooler in question. They can be, of course. They are not necessarily effective simply because it has been labeled or described "oil cooler".
