aussiebug wrote: ↑Sat May 21, 2011 1:32 am
I am pleased to see Piledriver and Rockerarm have introduced more than the simple Dino/synthetic question here.
It's a lot bigger question than "are synthetic oils better?"
ALL oils have improved out of sight in the last 40 or more years.
The problem is that we are trying to compare MODERN oils with the needs of our OLD engines. Modern oils are developed for modern cars, not 40 year old ones. Engines design has improved a lot in that time, and oil technology is trying to keep up.
So lets have a look at it.
Lubrication
VW used to recommend single weight oils which had to be changed with different ambient temperature ranges. Then in about 1975, when multigrades became reliable, they started recommended mulitgrades. They always stated in the Owner's Manuals that any good brand HD (that's High Detergent, not Heavy Duty as some would think) oil was OK, and this made sense as the car was sold all over the world so good oil anywhere would do the job - the engine was not too fussy about the brand so long as it was a good oil. Almost all 4-stroke engine oils back then had large amounts of ZDDP added - up to about 1400PPM.
Oil is supposed to prevent metal to metal contact in an engine - maintain a film of oil between all moving parts. But our flat tappet engines need more than just oil - they need an anti scuff agent to stop metal to metal contact in those high pressure areas like the flat tappets. To do this, oil engineers have long used an additive called ZDDP (a Zinc/Phosphorus compound). As luck would have it - this compound is also an excellent antioxidant, so it also helps cope with the corrosive combustion by-products that get into the oil.
ZDDP is sacrificial.
1. It works as an anti-scuff agent by decomposing with the heat of microscopic metal/metal contact and coats the contact area with Zinc, which eventually wears off and so the process repeates. This zinc coating reduces the wear on cams and tappets, but does not eleminate it completely.
2. It chemically combines with combustion by-products to produce less damaging chemicals in the oil.
So you need enough of the stuff in the oil to last until you change the oil again.
Modern engines need less or none because they usually have roller tappets, and the engine computers used these days control the combustion process much better than a carburettor or mechanical FI ever could, reducing the amount of combustion by-products significantly.
Cooling
Overcooling and undercooling are equally damaging. Ovecooling increases the viscosity of the oil and the engine is always designed to work with a set range of viscosities - obviously an over-thick oil will not flow as well as one which is "just right". Under cooling will increase the oil temp and reduce the viscosity, risking more metal/metal contact.
Another factor not often known about is that for ZDDP to work, the oil MUST be hot - I had a long discussion with a 30 year oil engineer at Castrol Australia who stated that ZDDP will not get to a high enough temperature at the metal/metal contact point to decompose and do it's job, unless the general oil temp is above 80c (176f).
Claims like "I use synthetic oil because the oil temp goes way down" might be completely anti-productive.
Summary
Oil manufacturers are reducing the maount of ZDDP in oils as it posions catalytic converters. Manufacturers are designing engines with roller tappets so they do not need so much (or any) ZDDP in the oil.
Many modern oils do not have much if any ZDDP, which our old engines still need.
Synthetic v Dino oils.
It doesn't really matter that much in terms of lubrication in general - both will do the job. They will both also carry engine heat to the oil cooler much the same. And both have changed there formulations a lot in recent years, so either could be right or wrong for our old engines. But I do not know of any synthetic oils which have significant levels of ZDDP (let me know if there are some).
The Castrol Engineer I spoke to said that despite the out-of-sight improvment in oil technology in recent years, our old engines will last longer if they have some ZDDP in the additive package. In Australia Castrol Edge Sport oils have a higher level of ZDDP than most - above 800PPM. But oil formulations around the world do differ, so we as owners, must determine if the MODERN oils available locally to us meet the requirement of our OLD engines. In many cases, certain oils are only available in certain countries - Royal Purple and Brad Penn in the USA, Duckhams in the UK, Penrite in Australia, and so on. And even the "common" oils like Castrol, Shell, and BP use different formulations in different countries.
So we have to consider the WHOLE deal - lubrication, viscosity, additive package - it's not enough to say "it runs cooler with this particular oil", or "I change my oil every 1000 miles to it doesn't matter what I use".
It's a little harder for us these days, because oil technoilogy has moved on from our old engine designs...we have to get a little more choosy ourselves, and to do that, we need enough knowledge to make the right choices.
I guess the average aircooled VW owner has to be smarter than the average bear
