Changing Your Car Oil
Changing oil is probably our least favourite maintenance activity, but is the most important of all for ensuring long engine life. Engine oil serves two major functions: lubrication and cooling. Moving parts rely on a very thin film of oil to prevent frictional wear, and the circulating oil absorbs a good amount of heat from the engine. This latter characteristic is demonstrated by the reduction in engine temperature when an oil cooler is used.
For proper lubrication, absolutely clean oil is essential. Since the lubricating film is extremely thin, any particulate contamination will severely interfere with proper lubrication. Indium coated engine bearings are particularly susceptible to mechanical damage from dirty oil. While a good oil filter will remove most particulate contamination from the oil, chemical contamination is not so easily removed. Chemical contamination consists largely of combustion by-products and resultant acid formation. None of us want to have acids circulating in our engines! It is because of this that the oil must be changed periodically.
Despite current claims that modern oils in modern cars need be changed at intervals of only 7,000 to 10,000 miles, our older British sports cars still require oil changes at least every 3,000 miles (or six months if the car isn't driven 3,000 miles during that period) for maximum longevity. If in doubt, follow the original factory recommendations regarding frequency of oil changes. One legitimate variation to the factory recommendations is the use of modern multi-viscosity oils with additives. The only defensible use of single viscosity non-detergent oil in engines is in old worn engines which have been run with older type oil. Modern type oil in these engines will often flush out accumulated build-ups of matter which worn engines sometimes rely on to keep them functioning as well as they can. A sudden release of these build-ups is not always beneficial.
If your owner's manual specifies only single viscosity oil, picking the correct multi-weight oil should not be difficult. For general use, 20W40 is generally a good choice for most engines. Consistent very high temperatures and/or very "hard" driving may require the use of a higher viscosity rating due to elevated oil temperature. Conversely, extremely cold conditions may require a lighter oil. Your mechanic or local car club can provide more detailed advice to suit your local conditions.
The actual process of changing oil is not at all difficult. Drain oil when the engine is warm into a large drain pan, replace and tighten the drain plug (and drain plug washer if one is used), replace the oil filter with a new one, and fill with new oil. Run the engine and look for any oil leaks. Shut the engine off, let sit for about 5 minutes minimum, and re-check the oil level. It may be necessary to add a bit more oil to reach the "full" mark on the dipstick. Do not overfill the engine with oil, as this may lead to foaming, which drastically reduces the oil's ability to properly lubricate your engine.
If you have a spin-on oil filter, changing it couldn't be simpler. Just be sure to follow the installation instructions, and don't over-tighten. The older canister type filters, with replaceable internal elements, are much messier and more difficult to deal with. This type has a separate sealing ring between the upper edge of the canister and the filter head, which fits into a deep groove in the filter head. Always replace your old seal with the one included with the new filter element. The old seal is often so hardened by age and use that it seems there is no seal there at all. A sharp ice pick or similar tool may be required to remove the old seal. Some cars, notably MGA and early MGB, have filter heads which are retained by the filter canister bolt, and have another similar seal between the filter head and the block. This seal should also be replaced. When disposing of the old filter element, don't throw away the plate in the canister - it often sticks to the canister!
With any oil change, it is essential that the old oil and filter be disposed of in a legal and environmentally safe manner. Many gas stations and garages will properly handle this final process for you, at no charge.
(Many thanks to Garth Bagnall for reviewing this article before it was printed)