Hi! You are welcome!
First, before You get too excited riding Your VN800, learn the rule number one. Never trust previous owner or Your inner feeling that everything should be OK before You are not absolutely sure that everything is OK.
The oil change is mandatory, but doing just some regular oil changes is not enough for this engine. Be sure to check the valve clearance as soon as possible! If the VN800 engine is not getting maintained every 6000 miles, it can end up with a dropped valve or even with a total engine disaster!
Second, try to find an owner manual on the Internet or even a service manual. (We are not allowed to share those copyright protected materials here.) You will find a maintenance schedule and other good stuff about daily checks and service procedures. You will also find that Your bike needs an ordinary (cheap and simple) engine oil which corresponds to API SE, SF, SG, SH or SJ with JASO MA classification. SAE 10W-40, 10W-50, 20W-40 or 20W-50 according to Your climate and outside temperatures. No even need to go for a pure synthetic, because in most cases it will be overkill. Most of them can have some friction reduction additives and it's bad for Your wet clutch. (I use an ordinary API SG 10W40 mineral oil for about 3 EUR per liter
And I found it being the best for now.
Yes, You can charge Your battery with 12V car or motorcycle battery charger. To be sure that nothing goes wrong, You can disconnect it from Your bike.
Don't hesitate to ask
Cobra pipes usually come together with a carb rejetting to get the correct performance. So, Your carb should be properly adjusted. If not, You will have a poor mileage.
Hopefully Your bike has no EPA system any more. If it has, Your MPG with Cobra pipes should be very low. It would be hard to get any fuel economy in this case