A. If you are using the older gasoline engine for normal operating conditions, a diesel engine oil type formulation does typically provide higher Zinc levels and the higher viscosity-15W-40 will provide extra oil film in addition to the higher Zinc. This answer is premised on the fact that older gasoline engines do not have modern emission/catalytic convertor hardware which is why the zinc and phosphorus levels have been lowered over the years to insure compatibility with the CC hardware.
If the engine is being used for a higher performance demand such as racing or more severe operation a true racing formulation is recommended which typically has higher zinc levels and higher viscosity.
Please provide more detail on the make, model and operating conditions and we can make a more specific recommendation.
In the diesel engine oil category, CITGO offers a high performance diesel engine oil called Mystik JT-8 SHD(Super High Performance) Heavy Duty Engine Oil 15W-40.