Cool thread! Thanks for the info!
ZDDP additive is great in moderation in my opinion. SSD already did all the hard work and told us exactly how much Lucas to use per oil change. I personally run a quarter bottle of Lucas ZDDP moose juice per oil change. Of course Lucas suggests 9 bottles per oil change. They love money.
15w-40 is hell on a car engine at start up even in 75 degree weather!
It is pure suicide on your engine at 30 degrees.
Running diesel 15w40 in an effort to have more zinc:
you are having severely delayed lubrication to the upper half of the engine (dry rubbing cams and journals, pushrods, rockers, valves).
The object on startup is to get full oil pressure to all extremities of the engine instantly (near as possible to instantly). The other object is to reduce parasitic drag and allow oil to return to the sump quickly while allowing foam and air bubbles to escape suspension in the oil.
Thick 15w40 may read full oil pressure on the gauge in 10 seconds, but the small parts are all still waiting (dying).
Engine oil will take 20+ minutes to warm up fully. During this delay, thicker oils will exacerbate the wear on engine extremities.
Thicker oil robbs horse power, clings to the crankshaft weights and throws, starves the engine on startup and fails to return to the crankcase quickly, foams more, traps air bubbles more and thins out more as it gets miles on it.
Racing oil at 40, 50, 60 wt is designed to operate at 300-500 degrees F. It thins out to its predetermined viscosity based on operating temperature and stays there until the end of the race where it is drained and chucked in the fire.
It is not designed for proper lubrication at startup, it is not designed to operate at 30F and is not designed to operate at 220 - 250 degrees. It does not have additive packages which allow for operation at 2500, 3500 or 5000 + mile intervals.
You only need enough "thickness" to maintain 10 psi per 1k rpm. Tailor your high side number accordingly (based on psi at max operating temperature). Then run your low side number as you can afford $. For example 0w40, 0w30, 0w20, 5w30 etc. This way you have optimized startup lubrication, reduced engine drag, better return to sump and more engine longevity.
If you choose to run rotella or similar for a higher detergent and zinc-phosphorous package, run 10w30. Zinc and many other additives are consumed as the oil is driven over time, in the engine which is something that was not mentioned thus far.
Lately I prefer NAPA 5w-30 synthetic with about 3 oz of LUCAS ZDDP in older engines. I run rotella or dello 10w30 or 15w40 w a bit of zddp in the farm equipment.
BTW, almost every "Synthetic" on the market are not synthetic. Just high grade, highly refined crude oil. True synthetic costs a fortune. Mobile 1 is not true pure synthetic. It may not even be synthetic at all.
Just look up the legalities behind the nomenclature "Synthetic Oil".
http://www.bobistheoilguy.com/
A wealth of knowledge if so inclined.
