if the Viscosity of a sample of Shell Heat Transfer Oil S2 was measured and found to be 25.7mm2/s @40C, is the change can effect on heat transfer, will this increase or decrees the heat transfer and how huge it is

A thinner fluid gives better heat transfer. You should understand why the viscosity is so much lower. Apparently, something has changed. The viscosity should not be lower than ISO VG 26. So I think it's time to change the liquid.