Modern engine lubricants often do not suit older vehicles since they are designed for fuel efficiency, emissions control and extended change intervals. Older vehicles were designed before the advent of this technology, and therefore they require oils specially blended to match their needs. When considering oil choices, the following factors should be borne in mind:
1. Viscosity
It is usually imperative to use a high viscosity oil in the lubrication of veteran, vintage and classic cars. This is because a) by modern standards the oil pumps are relatively poor and a low viscosity oil will show noticeably lower oil pressure; b) bevel gears and cross shafts will fling off a low viscosity oil; c) oil seals will be more prone to leak, and d) oil consumption will be noticably higher.
2. Anti Wear Additives
Historically, the anti-wear additive used in the majority of formulations was a zinc / phosphorous based compound known as ZDDP (Zincdialkyldithiophosphate). Unfortunately ZDDP has a detrimental effect on sensitive exhaust emission systems on modern cars, so it has been phased out by oil companies, while engine designs have been changed to accommodate the removal of ZDDP. These oils, while performing well in modern engines designed for them, can cause substantial and accelerated wear in older engines such as accelerated camshaft / cam follower wear.
3. Detergent
Detergents are incorporated into all modern motor oil formulations and have been since the 1940's. Their function is to maintain engine cleanliness and they are also useful in combatting the effect of acid contamination of the crankcase oil caused by combustion by-products. In cars with restored engines, the use of an engine oil containing some level of detergent will not cause any problems. Non-detergent oils can be used either when an unknown oil is being replaced or where there is a likelihood that sludge has built up and risks damaging the engine if it is circulated. Owners of vehicles without full-flow oil filtration frequently express a preference for non-detergent oils.
4. Transmission matters
When it comes to Gearbox oils, many older vehicles specified EP80 or EP90 weight oils. Because the viscosity of gearbox and engine oils are measured on different scales, an SAE 80 gearbox oil is roughly equivalent in viscosity to an SAE 20 engine oil. An SAE 90 gearbox oil is roughly the same viscosity as an SAE 50 engine oil. Most manufacturers now offer EP80W/90 oils in place of both EP80 and EP90. In viscosity terms, this is analagous to using a 20W/50 engine oil in the sump, and will do no harm. It is important to look at the classification of all gear oils when used in older vehicles. Some 'GL5' EP gear oils contain an additive package which can be harmful to yellow metals (eg phosphor bronze) and should be avoided if in any doubt, and a GL3 or GL4 oil used instead. The definitions of the API (American Petroleum Institute) GL numbers are as follows:
API GL-1, oils for light conditions. They consist of base oils without additives. Sometimes they contain small amounts of antioxidizing additives, corrosion inhibitors, depresants and antifoam additives. API GL-1 oils are designed for spiral-bevel, worm gears and manual transmissions without synchromesh.
API GL-2, oils for moderate conditions. They contain antiwear additives and are designed for worm gears. Recommended for proper lubrication of tractor and farming machine transmissions.
API GL-3, oils for moderate conditions. Contain up to 2.7% antiwear additives. Designed for lubricating bevel and other gears of vintage transmissions. Not recommended for hypoid gears.
API GL-4, oils for various conditions - light to heavy. They contain up to 4.0% effective antiscuffing additives. Designed for bevel and hypoid gears which have small displacement of axes, the gearboxes of trucks, and axle units. These oils are normal for later classic synchromesh gearboxes.
API GL-5, oils for severe conditions. They contain up to 6.5% effective antiscuffing additives. The general application of oils in this class are for hypoid gears having significant displacement of axes. Oils in this class, which have special approval of vehicle manufacturers, can be used in synchronised manual gearboxes only.
API GL-6, oils for very heavy conditions (high speeds of sliding and significant shock loadings). They contain up to 10% high performance antiscuffing additives. They are designed for hypoid gears with significant displacement of axes.
Thicker gear oils are available for non hypoid applications, in SAE140 and SAE250 viscosities. The SAE250 oil replaces a grade which used to be called 600W. Thicker than that and you are into semi-fluid greases, which are used in some gearbox applications.
