New To Me '03 Tahoe

['68OrangeSunshine]

So I just bought a 2003 Chevy Tahoe 2WD. I'm getting used to some of its foibles. Like completely blacking out when I tried to motor up the Drivers Side window after off-loading groceries. I checked the voltage with a Fluke meter and got 12.87 VDC. About 10 or 15 mimutes later the lights came back on and I could start it and get the window up. I turned the lights to Zero and let it idle to charge up for ten minutes.
VIN is:
1GNEC13V33R20XXXX

I caught a glimpse of a chart in the Owners Manual and thought the initial G was the engine code. I didn't want a 496 Cubic Inch BBC. But I looked on the wrong page. It has a 4.8 L Vortec [=292 CI]. Which is acceptable. A 5.3 [327] or 5.7 [350] might have been preferrable. Or is the 5.7L obsolete in this generation?
The only worry I have is the miles indicated on the odometer are over 295K.

The electric windows like to play tricks on me. Yesterday, on the way home after I bought it [from a private party] the RR window would not move, The LR window would only go down 2''. Today, on the way to the MVD, the RR window went down halfway. When I got there, I couldn't get it up. After messing with the switches on the door console, I found the Kiddie Lock had been engaged and then the RR window went up.


A couple of questions I would put to Board Members more familiar with this series:
What grade of motor oil should I use?
What is my rear end ratio?
Fuel tank capacity?
What transmission do I have?
What can I do about paint patches?