So, is it's OK to use synthetic oil again if th previous if previous owner used semi synthetic for about 4 years. Before him synthetic oil was used.

But his oil change intervals were 10k kilometers, which is less than its mentioned in service book (15k kilometers).

Do using synthetic oil again will not harm the engine?

Engine's still pretty fresh, 143k kilometers on the clock.

And if I use synthetic oil, then I should stick to a 15k kilometer interval?

And about the oil. Shell oil catalog says that I should use SHELL HELIX ULTRA AV-L 5W30. But I can get SHELL HELIX ULTRA 5W40 for a good price.

Which oil should I choose? Temperatures in Latvia can drop well below 0 degrees. So that should be a point to watch for.

Thanks!

Have a look for the "oil selection and technology" threads in Site Sponsors -> Opie Oils.

Both your quoted Shell products have the same 5 centi-Stokes viscosity at 0C cold/Winter charactoristic; the difference is that the AV-L has a 30cS viscosity at 100C rather than a 40cS viscosity. So the AV-L is thinner when the engine reaches operating temperature. Bizzare as this may sound, the thinner oil is probably more desirable given that your car has the 1.8T engine, and hence hydraulic tappets.

my skoda has an AGN non-turbo engine. Maybe helix ultra will be ok then? It's a lot cheaper than av-l. And is sold in 5 litre can instead of one liter cans.

Ah; I'd not realised you'd the non-turbo. The reason for using fuly synth oil in petrol turbos is because of just how hot (literally "glowing orange hot" [hotter than just red hot) the exhaust turbine wheel and bearing house can get.

Ah; I'd not realised you'd the non-turbo. The reason for using fuly synth oil in petrol turbos is because of just how hot (literally "glowing orange hot" [hotter than just red hot) the exhaust turbine wheel and bearing house can get.

Ok, then I will go off with 5w-40. Oh, and last question, so using synthetic oil if semi oil is used previously is ok then, yes?

I understand it is on these engines. The issue where it wasn't was with developing oil leaks on engines with natural seals.

I understand it is on these engines. The issue where it wasn't was with developing oil leaks on engines with natural seals.

On my Skodas engine theres actually an small leak in valve cover gasket. So it's actually quite a risk using syint oil. maybe..

Sorry if I'm getting irrational. I just need some advice.

That I can help with too. There are 2 usual causes of cam/rocker/valve (you'll meet all 3 names for the same part in English) cover leaks:-

1) Cover bolt(s) under-torqued. So check the relevant bolt(s) are done up right. Usually spin them down finger-tight, then use a spanner or socket to angle-tighten 60 degrees.

2) Actual break in gasket. Obviously, this requires the cover removing to check.

If neither of those are the case, the cover may be warped. You can check for warping by putting the cover in a piece of glass and seeing if it rocks around either diagonal. I'm afraid this is likely to require a new cover, which is why I've checked the cheaper things first!