tanny wrote:
What is the main difference between synthetic and semi synthetic oil (apart from the 'semi'), how does it impact on performance?
Which is generally better, what is better in winter?
Its a long tale and you should all check what type of oil your manufacturer states, however
From what I have read, mineral oil breaks down quicker than synthetic oil, this means your car needs to have oil changes more frequently.
The general rule is that with mineral oil if your main driving includes any of these below the oil needs to be changed at approx half of what the manufacturer recommends
Stop/start city driving.
Mountainous driving.
Towing
High temps above 30c and below freezing.
Short runs of less than 5 miles.
You could say that most of us do some or most of these, so changes at half the recommended interval is a good way of prolonging the life of your engine.
Synthetic oil is supposed to last twice as long as mineral, but it is twice the price. Semi synthetic is a mixture of both so is cheaper and still has some of the benefits of synthetic.
It is also recommended that you should change your oil at least once per year, if you only do low mileage say 3000 miles it is not really worth buying synthetic.
Don't forget gearbox, rear differential oil, brake fluid etc need to be changed also as they are all effected by time and usage.
The bottom line is if you intend on keeping your car for a long time then oil is a cheap insurance against major breakdowns.
Synthetic if you can afford it is best for high and low temperatures although 15w40 semi or mineral oil is good for down to -15C and up to +40C