jonmac wrote:
>
> Listers
> I know this thread comes up for an airing from time to time
> but I thought I'd post some 'off the record' comments that
> have come into Gaydon over the last few weeks from oil
> companies about modern oils - i.e. those which have
> synthetic properties in them and others which are full
> synthetic.
> There's been a bit of a hue and cry in recent months in the
> UK as to whether a 'synthetic' oil is suitable or better for
> a classic engine. [snip] While acknowledging that modern oils are overall
>better (?)
> than their forebears (they have to do a much more demanding
> job anyway) the oil companies I've spoken to at some length
> feel classic car owners will be doing themselves no favours
> by using these lubricants in older engines.
There are those who swear by full synthetics, but whenever this subject
comes up, I generally repeat the following:
When Mobil 1 was first on the market, I was working at a city
engineering department on a survey crew. The guy running the crew raced
snowmobiles and tried Mobil 1 5W-30 in his sled one weekend and burned a
bearing. Not sure if the engine was tired, anyway, he put the newer
engine from his brother's sled in his for the next weekend, filled it
will Mobil 1, and burned a bearing.
Not too long after, there was a letter in the technical tips column of
R&T, from a man in Germany who'd had his Mercedes 380SL rebuilt by the
factory (only hints about the true cost of that!), and shortly after
breaking it in, loaded it up with Mobil 1 5W-30, took it out on the
autobahn, up to about 130 mph, and promptly burned a bearing.
Much, much later, a fellow working for me, with a Pro Stock car, tried
Syntec 5W-50 at the drags one weekend... as he went through the traps
and let off the throttle, and best guess is that the oil in the pan went
forward suddenly, the engine lost prime to the oil pump and he spun a
bearing. Never a similar problem when he went back to 20W-50.
Both of these oils are full synthetics and have one characteristic in
common. They are, quite simply, 5W oils. Most people assume that when an
oil is rated 5W-30, it has the _viscosity_ of a 30W at operating
temperatures and pressures. In fact, it has the lubricating capacity of
a 30-weight oil, and there is a world of difference between that and
viscosity. The latter figure indicates the approximate shear and film
strengths of the lubricant when hot.
But, viscosity is critical for maintaining oil pressure, especially when
the engine is pushed hard. Mechanical forces go up by the square of
engine speed. Double the speed, and the kinetic energy of the parts
flying around is four times as much.
Full synthetics, because their viscosity is much lower (it's one of
their main attractions--easier starting in winter, and less friction)
simply run out of spaces such as between bearings and crankshaft much
faster. Therefore, it takes a higher-capacity oil pump to maintain full
pressure at the extreme ends of the oiling system. If pressure isn't
maintained, the thickness of the oil film is reduced, and the oil has
less ability to resist metal parts contacting each other. For those who
say, "I run synthetics and the oil pressure's great," it's worth
mentioning that, on Triumphs, the oil pressure sender is located in the
main gallery fairly close to the pump. The sender isn't telling you
what's happening at the bearings.
The synthetics have superior oil film strength and shear strength, but
not enough to make up for not enough oil at the bearings when the engine
is really cooking, unless all the engine clearances and the pump volume
are adjusted for its use. And, John Macartney's remarks about newer oil
additives attacking older non-metallic parts (i.e., seals, gaskets,
etc.) is quite true.
For people who have reasonably fresh engines, don't push the car hard,
there may be some benefits to full synthetics, but those benefits
probably don't outweigh the additional current and future costs.
Cheers.
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