Oils ,semi and fully synthetic

EireRoadUser

I have a diesel engine and am about to top it up.

Does it matter if i mix semi/part synthetic with fully synthetic?

Also do stp engine cleaning products have any affect on engine running??

Thanks in advance

BrianD3

I have never found a satisfactory to this question or many other questions on the topic of oils. To be safe, I'd prefer not to mix semi synths with full synths if possible.

What sort of car is it? Is there synthetic oil in it already or do you not know what was put in it last or is there another reason why you want to know if you can mix oils safely.

On the stp cleaning products - are you talking about stuff you throw into the fuel tank which is supposed to clean injectors etc. I haven't seen any evidence to suggest these products are any good in a petrol engine but it may be different for diesels.

EireRoadUser

I just threw in half litre of synthetic stuff ,I'm sure if they didn't go together someone would have said.

Asked a guy in halfords about the cleaning agents, he said they void manufacturers warranty.

While i'm here ,which oil is the one that engines like best ??

BrianD3

Don't listen to anything that anyone working in Halfords tells you. Halfords is the "PC World" of motor factors. I don't know if cleaning products void the manufacturers warranty but I bet the guy in Halfords doesn't know either and was just shooting his mouth off.

As for the oils - I would be 99% sure you'll be fine topping up with your 1/2 litre of synthetic oil but next time you need to top up find out what oil was used in the last service and/or what oil the car requires and use that. The type of oil that an engine likes best is whatever oil is recommended by the manufacturer. Brands of oil don't really matter, there is a lot of hype, heresay and marketing bullsh1t associated with oils. But what does matter is the ACEA and API rating, any other specific manufacturer ratings (eg MB 229) and the viscosity.

crosstownk

As stated above, you must be sure to match the specifications.

Most newer cars have longlife service intervals and if you put a lower grade of oil in you are now back to the 'standard' service interval for you model of car.
For example, if you have a VW with longlife service and you top up between service with a semi synthetic then your long life service is negated and you should have the car serviced 10000 miles after the last service. Lower grade oils can't last as long as the higher grade (among other things).
Your cars owners manual will tell you which oil to use and what standards to adhere to.

When in doubt top up with the best pssible oil available.

EireRoadUser

Thanks both of you for your replies .

I did read the manual ,but i don't speak in oilage so I wasn't sure.
I enjoy driving ,it's just my interests in depth are elsewhere.

I don't intend on coming accross in stuper ,sorry if thats the case .It was just I wasn't sure about the whole oil thing in general.

Thanks

I know whats meant alright about halfords being like pc world, I asked about becker units and the guy in the radio section sniggered at me "never heard of them".

BrianD3

I'd imagine that for a modern diesel you'll need an ACEA B3 oil. The manual will also state the recommended viscosity (10w40 or 5w30 etc.) Often there is a range of allowable viscosities. The manual may also state whether you need to use semi, full synth etc. If it doesn't state anything about this then just use a semi with the right ACEA rating and viscosity.