Engine oils-which one

[bush16]

whats the difference of part synthetic oil and full its for my avensis we have been usung 5w 30 part synthetic but noticed castrol is on offer at halfords

[ChrisT70]

dont buy synthetic if your engine doesnt need it. cheap synthetics at £17 a gallon are also not a true synthetic oil, they are legally allowed to use the word but its not 100% true. you willbe paying 40 quid a gallon for something like silkolene pro etc.costco selll mobil 1 at 23 quid a gallon and castrol edge (semi) at 29 quid for 2 gallons.

havoline and a few others do a decent cheap semi at about 12-15 quid a gallon.

semi synthetics are a mix of pure hydro cracked synthetic oil (small % age) and refined mineral oils. pure sythetics are made without use of base cheap oil and are basically for highly tuned engines /trackday use/sports cars, its not that often a daily runner would require a proper synthetic oil. always check the spec in the handbook and buy just what you need, nothing more, regular oil changes are better than going too long on a good spec oil.