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Oil Grades - Can comeone please explain to me.....

Right, I have been on and on and on and on about oils for the last week and there has been some great help on these boards but I need some more.

I went to get some oil today ready for the oil and filter change I am planning on carrying out shortly. I was going to get 5w40 synthetic oil but when I got there they dont do it. On the shelf they had :

Castrol GTX Magnatec Partially Synthetic - 15w40 and 5w30.
Mobil 1 Fully Synthetic - 0w40 and 15w50.

Now I am have no clue which one of these to get and what grade. I even looked at the handbook in Halfords and they stated that 15w40 was recommended by Renault by Synthetic 15w50 is the ideal oil for the 2.0 engine.

Can someone please help me as I just seem to be going around in circles.


  BMW 320d Sport

Get the Mobil 1 15w50. Its not a real problem if the 2nd number is higher by a bit. But if you go lower with the first number then its no good.

Ie. Renault recommend 15w40, so 15w50 is ok, but 10w40/5w40/0w40 would not be.

You must keep the lower number correct.
  BMW 320d Sport

On top of which if for some reason it knackers your engine (which I can guarantee 15w50 wont) then youd have a pretty good case that Halfords published data that recommended that grade of oil and sue them!
  Skoda Fabia vRS

i was gonna say, the grade you probably looked at was for the 2.0L Williams

the 172 shouldnt be on a 15W-50 until its done tons an tons of mileage

stick to a 5W......a 5W-30 or 5W-40 should be fine

Cheers Nick and RM.

This is what I mean, there are conflicting reports of what to use. This is harder than I thought.

BRUN, I thought it was for a Williams too but the year of manufacture was 2000 onwards which is the 172 isnt it ?

RM172, if you look again in the handbook, ELF ULTIMATE 5w40 is recommended for the 172 not the 5w30, the 5w30 is for all petrol engines, LPG and multi-valve only.

The handbook says 5w40 so I am going to get that but I couldnt see that grade in Halfords.
  Skoda Fabia vRS

o, an id say stick to the 5W-40 then, cos the last number is like the hot rating, so if it specifies 40, then 30, though should be fine, might be too thin at high temps

jus get down to your Shell garage and get the Helix Ultra, thats 5W-40

I was running 5w/50 fully synthetic - now Im on a part synthetic 10w/40 (Renault say 10w/40 or 15w/40). I really dont notice any difference - except that the oil pressure at idle is now lower with the 10w/40 oil .

Can somebody explain? I thought that as 10w/40 is a thicker oil, the oil pressure should be higher (?).