Hi,
Could somebody explain engine oil to me please?

I’ve just had my car serviced and the Skoda dealer recommends Shell Helix Ultra AV-L 5W-30. This is a fully synthetic oil.
The oil level lamp came on yesterday morning when I left for work so I parked the car, let it cool and checked the oil level. The oil was close to the max level on the dipstick and when I restarted the engine the light didn’t come back on. This happended again this morning so I rechecked, the level on the dip stick was fine and it didn’t come back on when I restarted the engine. I drove about 50 miles between yesterday morning and this morning without any warning lights coming on.

To be on the safe side I went to by a litre to top-up if needed and I was sold a litre of semi-synthetic 10W-40 by Halfords here in the Netherlands. The label on the back claims that it’s suitable for all petrol, LPG and diesel engines.

I know that 10W is twice as thick as 5W from servicing forks but I’ve not idea what the difference between synthetic and semi-synthetic is and if the 35 vs. 40 means much, something to do with operating temperature?

Will I damage my engine using semi-synthetic 10W-40 when the dealer seems to recommend fully synthetic 5W-30? And can I topup fully with semi?

Many thanks for all advice received!
Would I be better off getting this oil from Halfords? This oil is returned by the ‘suitable oil’ widget on their website and the car has had regular services.

Firstly, don’t add any more oil as by the sounds of it, it’s just a fickle oil pressure sensor.

I’d pay more attention to using oil of the same viscosity ie. 5/30w as opposed to whether it’s semi or fully synthetic, so personally I wouldn’t use 10/40w.

Multigrade oils allow different viscosities at different temperatures. 5/30w will give you a 5w oil when cold and 30w when hot.

When the engine’s cold with low ambient temperature the oil viscosity is critical as the oil pump has to deliver that oil to the rotating components (camshafts) at the top of the engine with oil from the lowest point (sump). How quickly this oil arrives at that point will determine levels of wear. This is why it’s important not to drive engines hard from cold. Similarly when the engine’s hot you don’t want the oil to run too thin as it’s lubricating properties will be limited.

Firstly, don’t add any more oil as by the sounds of it, it’s just a fickle oil pressure sensor.

Or something significantly more significant.

How many miles has it done? Oil pump? Big end bearings? etc. etc.

I has 105k on the clock. Never had any major issues apart from the distributor having to be replaced at 75k which is standard apparantly.
Front right wheel bearing had to be replaced.
It always starts fine even in winter at -5C.

It had a major service and MOT last Thursday.

Take it back to the garrage, they might just have knocked a sensor off?