The oil you need to use can be determined by the instruments in the car itself. The Murci has both oil temp and oil press gauges.
A: if on one of your back road trips, the oil temps never get above 235dF, then you can easily go down 2 SAE weights. If you oil temps never get above 270dF you can easily go down one SAE weight.
B.1: if you oil pressure gauge is reading more than (10 + 10,000/RPM) PSI when completely warm then you are running a thick enough oil.
B.2: If your oil pressure is reading more (20 + 20,000/RPM) PSI then there is a good chance you are running an oil that is thicker than you need.
B.3: until the oil pressure gets high enough to blow the pop off valve and pressure rises no more.
The only way I can get my Ferrari (recommended 10W-40 Shell UH) oil sufficiently hot to be above B.1 and to be below B.2 is when I have spend at least 20 minutes running on race tracks and the oil is about 300dF. This takes R-compound tires, I can't get this oil above 285dF with street tires (B S03s). With 10W-30 Redline I remain above B.1 (barely) while running the living snot out of the car in Texas summer heat on street tires--oil temp right at 285dF.
I suspect your Murci has sufficient oil cooling to enable you to operate one grade below recommended (maybe even two--depending on how you drive), but you must take responsibility to WATCH the TEMPERATURE gauge rather continuously. As long as the temps remain below 260 there is nothing to worry about 1 grade thinner, between 260dF and 285dF you need to watch carefuly, but there is no need to alter your driving style. Between 285dF and 305dF is where you should remain concerned as to whether your choice of one grade lighter is worthy, and adjust you riving style so that the oil returns below 285dF.
Why 285dF? This is my edumacated guestimate as to where one grade lighter oil has an HTHS reading equal to the spec grade oil at 305dF. For example: Redline 10W-30 oil has an HTHS of 3.8, the Factory spec oil has an HTHS of 4.2 at 305dF. When RL 10W-30 is around 285dF its going to have about that 4.2 cP of High Sheer viscosity that that 10W-40 oil had at 305dF.