out of interest, the focus needed the oil topping up for the first time over the weekend. the manual reckons it needs 5w30; we were at the gf’s parents’ and all they had at theirs was something like 30w50. i figured it would probably have been ok, but got some 5w30 from the garage on the way home and used that.

what do the different numbers mean and how much difference is there, ie. what if anything would have happened if i’d used 30w50 instead?

it is the weight of the oil hot and cold.

DO NOT USE THE WRONG ONE. some engines die quite quickly on the wrong oil.

It will work for a while, but in the end its bad news. If i use 20/50w in my old beetle then it sprays oil out the seals once its hot as the oil is too thin.

The W figure refers to the low temperature viscosity and the other figure refers to the viscosity at 100°C. The lower the figures the thinner the oil. Best to use the correct grade for the engine but if you can’t get the correct grade then probably safer to go up in grade to a thicker oil. Using a thin oil in an engine not designed for it will give you wear problems particularly in journal bearings. The 30W50 would have been OK to use as a top up.

More important still than the weight is the specification code eg VW50500 (or something). Don’t put petrol oil in a diesel, the rings will gum.

For beetles, the thermostat will not work if you use multigrade. The reason seals fail is because of oil pressure when cold, basically oil goes through the cooler when it shouldn’t. The problem manifests itself at higher temperature as you point out. HD30 is the only thing to use, HD40 if it’s very worn.

As above. For a top-up you’re safer putting ANY oil in than none at all. For a full change you want to get the right stuff. Ultimately you should be sticking to whatever grade the manufacturer says unless your car is tuned/tweaked greatly, in which case you should know what the values mean. Make sure it meets the required grades for anti-foaming and additives (noted as things like B4 etc on the rear of the tin). Oil is a very hotly discussed topic in car enthusiasts world, but most of the time its best to stick by the manufacturers words.