Mineral engine oil is refined from crude rather than synthesised, and it remains the practical, economical choice for older engines, high-mileage vehicles and equipment that does not call for a synthetic.

Typical grades are 15W-40 and 20W-50, along with monogrades such as SAE 30, 40 and 50 for older machinery, generators and some agricultural equipment. Higher-mileage engines with looser clearances often run better on a heavier mineral grade, which maintains oil pressure and reduces consumption compared with a thin modern synthetic.

The trade-off is service life. Mineral oil oxidises and thickens faster than synthetic and does not cope as well with sustained high temperature, so drain intervals need to be kept short and to schedule. Check the API service level in the handbook rather than choosing on viscosity alone.