Castrol R Advisory Letter (1961) regarding the BSA Super Rocket (A10)

Issued on 21 September 1961, this advisory letter from Castrol Limited offers a detailed and period‑authentic explanation of how Castrol R should be used in relation to the BSA Super Rocket (A10), one of the most charismatic British twins of the post‑war era. Written by the Motorists’ Advice Bureau at Castrol House, Marylebone Road, the letter responds to a technical enquiry from an owner and provides guidance that reflects both the engineering realities of the early 1960s and Castrol’s long‑standing reputation for specialist lubrication expertise.

The BSA Super Rocket, produced from 1957 to 1963, represented the high‑performance end of Birmingham Small Arms’ A10 line. With its alloy head, larger carburettor, improved cam profile and strengthened bottom end, the Super Rocket was designed to deliver strong acceleration and sustained high‑speed running. It was a machine that rewarded careful maintenance and correct lubrication, particularly given the stresses placed on its bearings and valve gear at elevated temperatures. Owners frequently sought advice on oil choice, especially when considering specialist lubricants such as Castrol R.

Castrol R itself occupies a unique place in motoring history. Derived from castor oil, it was prized for its exceptional film strength, thermal stability and ability to maintain protective qualities under racing conditions. Its distinctive aroma became synonymous with competition engines, from motorcycles to early racing cars. However, Castrol R also required careful handling: it could not be mixed with mineral oils, and engines needed to be flushed thoroughly before switching between oil types. These characteristics made it both revered and misunderstood, prompting Castrol to issue advisory letters such as this one to ensure owners used it correctly.

In the letter, Castrol explains that Castrol R is primarily a racing lubricant, but that it can be used for general road work in high‑performance machines such as the Super Rocket, provided the owner understands its limitations. The writer notes that Castrol R offers advantages under sustained high‑temperature running, where its film strength and stability outperform mineral oils. At the same time, the letter cautions that Castrol R should not be mixed with mineral oils under any circumstances, and that a proper flushing procedure must be followed when changing from one type to the other. This advice reflects the practical workshop knowledge of the period, when castor‑based oils were still widely used in competition but required careful management in everyday machines.

Accompanying the letter is the original Castrol instruction sheet detailing the recommended procedures for flushing the engine, gearbox and rear axle. These instructions outline warm‑drain procedures, the use of Castrol Flushing Oil, running conditions during the flush, and the additional steps required when Castrol R has previously been used. The sheet also includes a clear warning that vehicles should never be driven with flushing oil in any unit — a reminder of the practical risks involved in mid‑century maintenance. Only the upper portion of this sheet is shown in the listing, but its presence confirms that the advisory packet remains complete.

Taken together, the letter and instruction sheet form a coherent and well‑preserved example of mid‑20th‑century technical correspondence. They offer a direct insight into the concerns of performance‑motorcycle owners, the behaviour of castor‑based lubricants, and the customer‑support practices of one of Britain’s most influential oil companies. For collectors of petroliana, British motorcycles or technical documentation, this is an unusually complete and historically resonant artefact.

Condition Report

Letter: Very Good — clean and legible throughout, mild age‑toning consistent with early‑1960s paper stock. Original folds present but stable. No tears, losses or oil staining. Instruction Sheet: Very Good — clear text, light toning, original folds intact. No tears or significant creasing. Provenance: Complete advisory set as originally issued, with no modern marks or additions.