You are not going to like this thought. The increase in temperature might be from internal engine friction with too thin an oil. I doubt the rise in temperature is the air leak around the injector.

I hope my thought about the too thin oil is wrong. It might also be that the synthetic oil is not removing heat from the engine as well and the cooling system is working harder.

A small percentage of us really old guys are reluctant to change the type of oil used after wear patterns have been well established as well. Just call us stuck in the mud if you wish. To do so and reduce viscosity at the same time below values that have worked for years does not seem like a sound move to me.

Even this far north again we always want to see a 40 weight or higher number in the multi grade formula. Never smaller. Call it old wifes tales again if you wish.

As costly as it is I might consider dumping the new oil into a storage container and get something else more appropiate in that engine. Too expensive of a potential gamble otherwise. Or it would be to me.