Something to keep in mind about multi-viscosity oil. For conventional oil, the multi-viscosity spread is achieved thru additives, usually polymers (little chains that curl and uncurl with temperature changes) that are susceptible to sheer and oxidation. So those additives give you good properties of multi-viscosity, but they can contribute to deposits/dirt in the engine and also degrade the oil as they wear out.
For that reason, its a very good suggestion to go with the smallest possible viscosity spread that you need for the conditions. 10W-30 is less likely to leave behind deposits and probably last longer than 10W-40 will under the same conditions.
But then again, if the conditions you drive in, actually require 10W-40 then 10W-40 is going to protect better than 10W-30, and your motor will be better off even if 10W-30 would leave behind slightly less deposits.
Thats why I believe an oil pressure gauge is so important to an vehicle. It will give you good indication as too whether your using the proper viscosity oil or NOT, and you can pick the proper viscosity oil for the conditions you operate the vehicle.
So if 10W-30 will meet your needs, its probably better to use it over 10W-40. If oil pressure is too low for a warm engine under load in the summer, then maybe 10W-40 is called for.
Flushing Radiator? What type of anti-freeze is in it now?? Conventional Green Anti-Freeze is recommended to be changed every 2 years. The anti-corrosion additives is the first to deplete in anti-freeze and rust and corrosion can form on the radiator and other parts and create problems. Flushing does NOT need to be Powerful or Harsh with chemicals. If there is nothing wrong with the cooling system, no deposits, no gunk, then simply drain it completely (that includes removing the plug in the engine water jacket and disconnecting a heater hose and blow/suck the coolant out of the heater. Then fill with distilled water and run the motor to circulate all the distilled water, it will have to warm up to open the thermostat, let all the fresh distilled water wash out anything left over. Drain fully again and fill with a 50/50 mix of distilled water and anti-freeze.
Distilled water is sold at most supermarkets for $0.50 a gallon, its water with all the minerals removed, as close to pure water that you can practically get. Tap water usually has a lot of minerals in it, and those minerals boil out and form scale on the radiator and cooling system. So using distilled water keeps the cooling system clog free and working better longer.
Many manufacturers have now replaced their conventional GREEN anti-freeze with new Extended Life Universal Anti-Freeze. If you read the fine print, they say you have to completely drain the cooling system and flush it, before using this anti-freeze. Because its really Dex-Cool dyed green. There is a controversy if this actually good for your vehicle, the safest bet would be too use the old conventional anti-freeze you motor was designed for and change it every 2 years. Look for a green anti-freeze that is NOT extended life, and in the ingredients is list "SILICATES" and some form of "PHOSPHATE". That is the old green anti-freeze. So far, only Zerex still makes it, I've only found it still sold as a generic brand.
While your at it, you should replace the thermostat and pressure cap.