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Whats Difference Between Semi Synthetic And Full Synthetic

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manual recommends semi 5w30 just wondering what is the difference between semi synthetic and fully synthetic engine oil 5w30 is it that the fully synthetic is thicker oil and its that bit slower to get around engine from cold start than the semi what exactly is the difference and what difference could it make to my car, im thinking there must be some differance involving the running of an engine

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Semi synth has miner oil base and synth oil in it!... the problem is you never know how much synth is in the stuff. Full synth oil is usually dearer and have higher protective qualities but still get round then engine the same as semi. Oils like Castrol EDGE and Mobil 1 are something called a "Cracked hydro carbon" so not a true synth really, very good oils with high VI numbers and Flash points.

If you really want the top end oil then you have to look at double ester oils like Motul 300V ot the Gulf comp oils. Both have very high Total Base Numbers for anti-wear properties... But they are not always sutable for all engines as they tend to have a high ash content and no good for cars fitted with DPF...

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Fully synthetic oils are not really necessary unless you have a high-powered sports/luxury car or an engine with significant performance upgrades (highly tuned). Obviously engines that are subject to high stress levels (like supercars or motorsport machines) need the best protection available, so it makes sense to splash out for the finest oil. But the majority of road going cars are not highly stressed or severely punished on a regular basis, so semi-synthetic is more than adequate.

Older engines with high mileages tend to use up fully synthetic oils & burn them off, so can need regular top-ups. That's why semi-synthetic is more suitable for them.

I know this both from personal experience & a very respected race engine builder.

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