Classic engines and modern engines are different machines, and they need different oils. The flat-tappet pushrod architecture of the B-series in the MGB, the A-series in the Midget, the C-series in the MGC, and the XPAG and XPEG in the MG TD and TF was designed around the lubricant chemistry of the time, relatively low detergent content, high zinc-and-phosphorus additive levels for boundary-layer protection on cam lobes and lifters, and viscosity grades suited to the bearing clearances and oil-pump design of the era. Modern oils, formulated for tight-clearance hydraulic-lifter engines with roller cams, are not the right product for these engines over the long term, and this range exists to give classic owners the lubricant chemistry their cars were actually built around.
Why Classic Specification Matters
The headline issue is the zinc dialkyl dithiophosphate, or ZDDP, content. ZDDP is the additive that forms the sacrificial boundary layer between the cam lobe and the flat-tappet follower on cold start, and on a classic engine, where the cam-to-follower interface sees enormous pressures and almost no hydrodynamic film, that boundary layer is essential. Modern oils have had their ZDDP content steadily reduced over the last twenty years because the additive is incompatible with the catalytic converters fitted to modern cars, so a classic engine running modern low-ZDDP oil can lose cam-lobe material over a few thousand miles of use, particularly during cold-start running. This wear is insidious, not announcing itself with noise or vibration until the lobes are significantly worn, at which point the camshaft and all followers must be replaced.
The Castrol Classic range maintains the high-ZDDP additive package the engines were designed for, along with the balanced detergent content that suits an engine carrying decades of established deposits, as an overly aggressive modern detergent package can dislodge accumulated sludge and carry it into oil galleries and bearings.
The Castrol Classic Range
The range covers the full Castrol Classic line, monogrades in the SAE 30, 40, and 50 grades originally specified for the earliest pre-war and immediate post-war engines, multigrade 20W/50 suited to the MGA, MGB, Midget, MGC, and contemporaneous classics, and a castor-based high-performance oil for competition or spirited use, with a fully synthetic high-performance option for race and hard-road use and a dedicated running-in oil for the first fill of remanufactured engines. The viscosities are chosen to suit the bearing clearances and the oil-pump capacity of each generation of engine, as running a modern thin oil in a classic with the wider clearances of period design will not maintain hydrodynamic film pressure across the bearings.
Application Coverage
The MGA, MGB, Midget 1275, MGB GT V8, and contemporaneous A-series, B-series, and Rover V8 engines are all suited to the 20W/50 specification, the modern equivalent of the original handbook recommendation, while the MG TD and TF with their XPAG and XPEG engines are typically run on a 20W/50 multigrade or an SAE 40 monograde depending on operating climate, and the MGC's C-series six-cylinder also takes the 20W/50 multigrade. Pre-war and immediately post-war engines are typically run on the appropriate Castrol Classic monograde, and historically many motor manufacturers including MG recommended Castrol by name in their original vehicle handbooks, so today's owners can follow those original recommendations using modern formulation technology. The recommended oil change interval is typically every 3,000 miles or annually, whichever comes first, significantly shorter than modern intervals, and for cars used infrequently the time interval matters more than the mileage, as moisture and acidic combustion by-products accumulate in the oil and attack bearing surfaces even when the engine is not running.