Engine Oils.

The first number with the W is the cold-temperature rating — lower flows better when cold. The second is the viscosity at operating temperature. So 15W-40 is thicker when cold than 5W-40 but behaves the same when hot.

Using an oil that is too thick when cold delays oil pressure at start-up, which is when most engine wear occurs.

Generally not. Plant and agricultural engines often need specific API or ACEA specifications, and many manufacturers require oils meeting their own approvals. Some machines also use the engine oil for wet brakes or transmissions.

Always check the operator's manual for the required specification rather than choosing on viscosity alone.

Mineral oil is refined crude. Synthetic oil is engineered for more consistent molecular structure, giving better cold flow, thermal stability and longer drain intervals. Semi-synthetic sits between the two.

Older engines designed for mineral oil do not always benefit from synthetic, and in some cases can develop leaks as deposits are cleaned away.

Unopened and stored in stable conditions, typically several years. Opened containers deteriorate faster through moisture ingress and oxidation, and drums stored outdoors draw water through the bung as they breathe.

Store drums under cover, on their side with the bungs at nine and three o'clock, or upright with a cover to keep water off the top.

Modern multigrade oils cover a wide temperature range, so seasonal changes are rarely necessary in the UK. Machines working in extreme cold may benefit from a lower W rating.

The bigger seasonal issue is condensation in stored equipment, which contaminates oil regardless of grade.

Mixing approved oils of the same specification is generally not harmful in an emergency, but it dilutes the additive package and may compromise the properties you paid for.

Mixing mineral and synthetic, or oils of different specifications, is best avoided outside of topping up to get a machine to a service.