A MX-5 club member (thanks Morrin) has provided the following information which was shared with him recently when he went to replace the oil in his 54 MG, which he thought might be useful to other club members. Owners of MX5s over 50000kms should think about what oil they use, in particular NAs and high mileage NBs. Click on the link below to view the full info.
Why do hobby cars need special engine oil rather than the latest products on the market?
Modern engine oils are formulated to meet the requirements of the latest designs and also the fuel consumption and emission targets laid down by the legislators in many parts of the world. To meet these targets the latest engine oils are very light viscosity multigrades which whilst being suitable for modern engine designs, are not suitable in earlier veteran and historic engines.
These cars were originally built with different technologies and tolerances to late model vehicle. They also spend the vast majority of their time idle. Whilst a non-working engine is not wearing itself out, unprotected surfaces such as cylinder walls, camshafts, etc can be subject to corrosion.
Modern thin oils designed to circulate quickly through an engine will drain away from the internal surfaces back to the sump leaving little protection to the parts above the oil level and therefore prone to corrosive attack.
Continue reading here : https://www.penriteoil.com.au/tech_pdfs_new/Nov2015/Veteran_Vintage_&_Classic.pdf