API SM vs SL... and my car is a 1991, ZDDP levels?

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1) I am wondering is ZDDP is a GOOD or BAD thing... I have read about it, and im not sure what it is.

2) I am wonderign if my 1991 car NEEDED or DIDNT need ZDDP.. is ZDDP "Good" or "Bad" for it?

3) Im wondering the "ZDDP" Levels in API SM and SL.. Mobil 1 HM is SL, and Valvoline MaxLife is SM. Neither is Energy-Conserving.. Thats fine.

Im also wonderign is someone can show an example of a car that NEEDED an Oil high in ZDDP.. cars from the 70s, maybe?

Thanks :)

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Yes it is ok with SM oil.

Older cars and many racing engines need the extra ZDDP for the none rollerized camshafts, non roller over head cam motors don't have the valves spring pressure that a OHV motor has so the ZDDP is not as much of a deal with those.
 
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.
 
SL usually has around 1000ppm of Zinc, SM has under 900. Look at the VOA's on here for German Castrol (Syntec 0w30 SL oil) and the newer SM oils to see just how much.

Older V8 motors are usually the best example. A good 350 small block chevy like the 1970 LT1 motor is a good example of a motor that needs extra zinc or just about any Big Block Chevy with a non roller camshaft.
 
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