HOW FAR'S SAFE BETWEEN DRAINS?
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ALL SORTS of phenomenal between-service mileages have been quoted for oil changes. The point to stress here is that the idea of extending these periods does work, but it has got to be done properly.
Most of the advantages are indirect. It is nonsense to say that doubling the time period between oil changes will halve the bill. What the engine loses during its normal operating period is usually larger than that used in a complete change.
Extending the oil drain period means, in effect, halving the number of times the sump is fully recharged. Around 10 per cent is a reasonable saving figure for the lubricant alone.
The main saving is in the direct labour charge, but even this is not a direct financial saving because usually the vehicle has come in for more th.an just an oil change anyway and the same staff level will be maintained. It does mean, however, that the fitter has more time to do the rest of the tasks on his maintenance sheet.
The extended oil drain period can develop into a swings and roundabouts argument. Some operators have said that they don't change the oil for 100,000 miles. But as they have a high oil consumption so it can be argued that they are changing it regularly anyway. Others say that they change it at anything from 4,000 to 25,000 miles with nil oil consumption on the way.
Castrol realised some years ago that this was a fact of life with operators wanting extended drain periods. "We could see it coming and so we armed ourselves," the company said.
The people at Castro' agree that it can be done success
fully but add that it must be done under controlled conditions. It must be in a properly organised fleet, monitored throughout.
One big bogey which can occur is with fuel dilution of the oil which can result in a fairly dramatic drop in the oil viscosity. For example, an injector pipe splitting on an engine whose design has the injectors themselves situated under the rocker cover can cause a lot of expensive trouble before the problem is spotted.
The engine manufacturers argue that if you are changing the oil prudently, then this fuel dilution should never reach serious proportions. But the fact is that it could with an extended drain period. Hence my point that the idea should be tried only in a well organised fleet which would pull up any untoward leaks on the engine before damage was done.
Castrol recommends that the extended oil drain concept should be prudently used on indirect injection engines. Because of their combustion chamber design, the products of combustion tend to load the oil with soot.
The old pitfall of trying to relate mileage against time crops up in this context. With a vehicle which spends a lot of time with the engine running but for less time with the wheels turning— municipal vehicles for example—. it is not possible to rely entirely on mileage. Cas
trol says that it is necessary to tread warily in areas like this.
As a matter of interest, Castrol has achieved mileages without an oil change of around 70,000 on field trials. This was, of course, under carefully controlled conditions by way of an experiment and should not be taken as a recommendation. However, it does give an idea of what can be achieved under careful supervision.
So if you want to extend your fleet mileage between oil changes, then get professional advice first. It can be done, but you've got to be careful.
• by the Technical Editor