Viscosity: more or less?
Part two: viscosity that's too low or too high, operating temperatures, clearances in friction pairs, and the eternal “5W-50 or 0W-30” question.
This is the second article about oil viscosity (below is a link to part one). The thing is, car owners have asked a great many questions, both on the site’s forum and by email. And most of these questions come down to the fact that manufacturers often allow several viscosity options, while the opinions of oil sellers, and even of respected mechanics, often run completely counter to the manufacturers’ recommendations.
With all that in mind, I decided to write another article on viscosity, and I hope it will bring a bit more clarity to the question.
Discuss the viscosity articles on the forumPart one: everything you need to know about engine oil viscosity
- Too-low oil viscosity Why oils with reduced HTHS viscosity (ACEA A1/B1, A5/B5, now also C5 to C7) endanger engines not designed for them: a thin film, burning and risk of seizure.
- Too-high oil viscosity What happens when oil viscosity is above the carmaker’s norm: the engine runs hotter, parts wear faster, the oil oxidises, and it shows after 100–150,000 km.
- Oil viscosity at working temperatures How engine oil viscosity changes in a warm engine, and why two 5W-40 oils can behave differently, so carmaker approvals and ACEA classes matter.
- Engine warm-up and oil viscosity Why cold oil loads the engine: clearances are designed for 100–150 °C, and every warm-up in hard frost takes about 300–500 km off a new engine’s life.
- What are clearances in friction pairs To start with, a rhetorical question: is the diameter of the piston (assembled with its rings) the same as the inner diameter of the cylinder? Of course not!
- 5W-50 or 0W-30? What too-high and too-low viscosity does to the oil film between piston and cylinder, and what a modern engine risks on 5W-50, or an old one on 0W-30.
- In brief: viscosity, only what the carmaker recommends Engine oil viscosity must match the carmaker’s recommendations: both a thicker and a thinner oil shorten an engine’s life.
- 0W-20, 0W-16, 0W-8: is the oil too thin for your engine? Where 0W-16, 0W-12 and 0W-8 oils came from, what HTHS is, and why such oil must not go into an engine its carmaker did not design for it — nor the other way round.