1) Answered above, but basically cold viscosity and warm viscosity (5W-30) good for cold climates. (10W-40) better for warmer climates. (5W-20) is what most new cars are taking now due to the slight improvement in fuel economy it provides.
2) Synthetic is made from man-made materials, whereas conventional oil is made from fossil fuels and naturally occurring compounds. Synthetic blend is a blend of the two. Synthetic blend is a waste of money since you don't know how much of either type of oil you are getting. Either go with a quality dino (conventional) oil and change it every 3-5k or go with a quality synthetic and extend your oil change intervals a little longer. Most manufacturers (not dealers) recommend every 7500 miles regardless of the type of oil you use. The exception would be severe use as described in the owner's manual. Color of the oil is NOT a good indicator of when to change the oil. Just follow the guidelines in the owner's manual.
3) Toyota branded oil is made by Mobil. They do now have a branded synthetic 5W-20. Toyota branded oil is overpriced. I try to purchase the same brand oil if economically possible, but with the ridiculous oil prices these days, I often go with the current sale brand. I do always stick with synthetic. That's from my personal experience.
4) Yes. Over 150k is usually considered high mileage when it comes to "high mileage" oil.
For a Toyota in general, not so much.
5) Use the jack stands, chock the wheels, check your jack stands twice before crawling under. Wear latex gloves to keep the used oil off your skin. It contains carcinogens. Take your time and like mentioned above, ensure you are removing the proper drain plug. Your oil should be brown. It should NOT have any reddish coloring. When removing the filter, ensure you get the old gasket off the mating surface. Lubricate the new oil filter gasket (Toyota filters come lubricated but I always put on a dab more). And, only tighten the filter hand tight per the instructions (usually 1/2-3/4 turn past first contact of the gasket to the mating surface.
Do not overfill with new oil. A little low is better than a little too much. Take your time to fill to the proper level. Check the rest of the fluids while you're already dirty and under the hood. Do a visual check of the suspension components and exhaust system while the car is raised. Also, look for any signs of other fluid leaks. That's it. Most of all,
enjoy the satisfaction of working on your own vehicle. Have a beer or cocktail when you're done and admire your work.