As for fuel economy, i ran SAE30 mono grade oil for 10000km, and i found that i lost nearly 50km off per tank on my weekly refill. on the next oil change with 5W-40, I got back that 50km extra per tank.
The only difference is start up viscosity. SAE 30 has a 40C rating of 98cst. The shell 5W-40 is 74cst at 40C.
That alone made the fuel economy difference on start up.
And the higher the oil pressure, the higher the fluid friction and lower the economy. Thats why cars these days use 5W-20 or 0W-20.
As I said before, if your engine clearances are still tight go for lower viscocity, otherwise go for 20W-50.
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