Good ideas and Questionable idea
1) The weight reduction can only be a good thing. Note that there will be more variation in MPG due to outside temperature differences than what 40 lbs would cause in MPG. However, there is a difference and that counts.
2) It is not a good idea to change from the designed oil viscosity. The bearing tolerances and engine design are based on a specific viscosity and there is no "gain" to be had by varying what the design engineers spent there career getting right. Mobil is concerned with selling their oil. Think of what claims must be said to convince owners to change to Mobil Oil, and then think of what PROOF they provide that this is true for your Prius. Rather than changing viscosity, go to a synthetic 5W-30 oil until you find that Proof above. (Synthetic oils are what is used in aircraft engines to withstand the intense temperatures.) Additionally, my '99 Camry had some oil sludgeing problems and the fact that I had used the proper oil ensured that the warranty was honoured. (http://www.corolland.com/sludge.html for info.) If you have a problem and it is documented that you were using the "wrong" oil, a bad dealer will take advantage of this. Also, record your oil changes somewhere on the car with a permanent sticker if you do your own oil changes.
3) You will not accomplish anything other than blowing a fuse, breaking something or causing a fire by trying and force current into the 12V port....and I want you to be completely safe. The High Voltage to 12V converter in the Prius only operates in one direction. This converter is used to power the 12V system (lights, relays, windows, etc.) and to recharge the "12V aux battery". The primary function of the 12V battery is to engage the main battery relays to start the car from the HV battery and then let the HV to 12V converter power the 12V components. This aux voltage is regulated to keep the 12V battery topped off. Bottom line here is that your idea is well intentioned, but the High Voltage battery is only charged from the engine or regenerative braking. [One of the reasons for this setup is to ensure that in a crash or similar event, the HV voltages can be isolated at the battery and the rescue personnel/vehicle occupants are isolated from the high voltages].
4) Please keep thinking of way to improve things. If you continue, sooner or later you will have something really worthwhile.
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