1. It can be a good thing, but can be bad for catalytic converters. It provides last-resort protection against metal-to-metal contact when the oil film fails.
2. Check your owner's manual. My 2006 Camry's manual states that it requires ILSAC (and, I think API, though I'm not 100% and am going from memory here) approved oil. Pretty much all oils today meet API and ILSAC requirements, so I wouldn't worry about it.
3. Good question. I'm honestly not sure. One can do an elemental analysis (part of most UOAs) to find out how much zinc and phosphorous there is, but I'm not sure how to translate that into the percentage of ZDDP.
ILSAC and API both consider modern API SM/ILSAC GF-4 oils to be fully backwards compatible, though there may have been some controversy. See this quotation from the Wikipedia article on motor oil:
Quote:
All the current gasoline categories (including the obsolete SH), have placed limitations on the phosphorus content for certain SAE viscosity grades (the xW-20, xW-30) due to the chemical poisoning that phosphorus has on catalytic converters. Phosphorus is a key anti-wear component in motor oil and is usually found in motor oil in the form of Zinc_dithiophosphate. Each new API category has placed successively lower phosphorus and zinc limits, and thus has created a controversial issue obsolescing oils needed for older engines, especially engines with sliding (flat/cleave)tappets. API, and ILSAC, which represents most of the worlds major automobile/engine manufactures, states API SM/ILSAC GF-4 is fully backwards compatible, and it is noted that one of the engine tests required for API SM, the Sequence IVA, is a sliding tappet design to test specifically for cam wear protection. However, not everyone is in agreement with backwards compatibility, and in addition, there are special situations, such as "performance" engines or fully race built engines, where the engine protection requirements are above and beyond API/ILSAC requirements. Because of this, there are specialty oils out in the market place with higher than API allowed phosphorus levels. Most engines built before 1985 have the flat/cleave bearing style systems of construction, which is sensitive to reducing zinc and phosphorus. Example; in API SG rated oils, this was at the 12-1300PPM level for zincs and phosphorus, where the current SM is under 600ppm. This reduction in anti-wear chemicals in oil has caused pre-mature failures of camshafts and other high pressure bearings in many antique automobiles.