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choosing the right fluids: the specification is not optional

the fluids aisle is deliberately confusing, and the single most useful thing to know is that viscosity is not the specification.

engine oil

"5W-30" describes how thick the oil is. it says nothing about what is in it.

what matters is the approval printed alongside: the manufacturer's own standard, or an API or ACEA rating. an oil that is the right viscosity and the wrong specification can lack the additives your engine needs, and on some engines that means accelerated wear on parts that are expensive to reach.

this matters most on turbocharged engines, on diesels with a particulate filter (where the wrong oil ash content clogs the filter), and on anything with variable valve timing, which relies on oil pressure through fine passages.

your manual names the specification. it is worth reading once.

transmission fluid

the one to get right, because the wrong fluid destroys a transmission.

automatic transmission fluids are not interchangeable, and "universal" fluids with a long list of claimed compatibilities are a compromise at best. some transmissions require a specific fluid and nothing else works properly in them.

if a shop cannot tell you which specification they are putting in, that is the moment to ask.

coolant

colour tells you nothing. there are several incompatible chemistries and they are sold in overlapping colours.

mixing incompatible coolants can produce a gel that blocks the radiator and the heater core. if you do not know what is in there, the safe move is a full flush and refill with the correct type rather than topping up with a guess.

brake fluid

hygroscopic: it absorbs water from the air over time. water lowers the boiling point, and brake fluid that boils gives you a pedal that goes to the floor under heavy braking.

most manufacturers specify a change every two or three years, and it is one of the most commonly skipped services in existence. also, DOT 3 and DOT 4 mix, DOT 5 is silicone and mixes with neither.

power steering and differentials

both have specifications, both are ignored, and both are cheap to do compared with what they protect.

what we do

use what the manufacturer specifies, and tell you which it was. if you are looking at an invoice from us and want to know exactly what went in, ask, and you will get the actual product rather than "oil".