Not at all. Modern engines are designed to run on an oil with very low viscosity (thin), such as 0W and 5W. These are too weak for older engines, which have been designed to operate with a viscosity of 20w in winter and 30 or 40 in summer.
However, our later formulations, Pistoneeze and all our multigrades are detergent oils. Detergents keep the oil clean by holding the products of combustion in suspension in the oil and prevent the formation of sludge in the sump. It is possible to use these types of oil (of the correct viscosity) providing you are sure that the engine is clean inside and does not have any build up of sludge. This will be the case if it has been freshly rebuilt or is already running on a detergent oil.
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For other related useful info please refer to our FAQs Classic page.