Hianbo:
You are basically working on the correct concept. You are trying to pump out 65 gal/min of crude oil and cooling it down by 40 oC. You have the resource of atmospheric air at 35 oC and you could use it to effect the required cooling.
Your flow rate calls for a 2” discharge line on the required pump and I would use the pump to take the 90 oC crude and pump it through a bank of 2” finned pipes. I would employ electric drive axial fans (similar to those used on cooling water towers) to force atmospheric air across the bank of finned pipes. Since you have to use a pump, this infers you have electricity to drive it and, therefore, electricity to drive the fans. You might be hard pressed to cool the crude within 15 oC of its temperature, but it certainly is worth a try since you probably have little or no options.
You probably will require extra horsepower to overcome the inherent pressure drop through the finned pipe header, but this is a price you have to pay. This forced draft atmospheric cooler can be designed by you or you can specify and buy a unit already pre-fabricated from a local designer/fabricator.
This should be an easy and straight-forward project to design and implement. You would be very foolish and naïve to expect natural air convection to be sufficient. If you want to succeed with this application, you will have to invest in forced or induced air convection. I know. I have designed and worked with hot (90 oC) Ecuadorean crude in the Amazon jungle as well.