I realize this oil is thinner than the recommended 5w30, especially for the 2000 to 2004 S40 models, but I would also like the way a thinner oil produces slightly better fuel economy in the engine yet still being a good quality oil. I have a good stockpile of this oil for my other car and was wondering if anyone has tried and used this oil without any adverse effects, and maybe some positive outcomes as well?

Also on the side, Volvo says don't change the oil until every 5,000 miles or 12,000 KM, is that true? Because of the Turbo Charger? Or should I increase my change intervals to 3000 miles or 5,000 kilometers?

 

first of all - do NOT use 5w20 oil for your car if you drives aremore than 20 miles long.
When engine heats up, oil should retain its viscosity to stick to engine inner walls, producing a thin oil-film on surface. If viscosity is too low, oil becames kinda like a watter and material rubbing wipes the oil-film off. This produces higher material obstruction and heats inner wals more, than it should. Consequently, heated material expands, whish means gaps become tighter, which produces more material rubbing.
In short - you may drastically shorten engine lifetime using this low-viscosity oil.

Use 5w30 or 5w40, or 10w40, full-synth oil.
Take a llok at your oil after oil change. If oil is still clear after 1000 miles, it is obvious oil is of BAD quality. Oil must clan engine and therefore it must take all dirt, so good oil is dark after 1000 miles.
Like a soap - if you wash your hands and washing water is claen, it means all dirt remained on your heands.