Some points to consider:
- Syn blends have a content of synthetic base stock of varying percentages. IIRC, some may be as low as 10% or so, so the performance is not measurably better than pure mineral based oil.
Bottom line? Buy the full syn formula to get the best benefits.
- Castrol and other use a cheaper synthesized base stock called a Group III, which is hydroisomerized, crude oil derived, lube stock. While pretty good from what I know, it is not quite as good as a PAO, the type found in Mobil 1 and others.
Bottom line? Mobil 1 is best, but for street usage, any synthetic should do.
- If a car is well maintained, switching to synthetic should not be a problem at, say 80K miles. The only recommendation I would give is to monitor the oil level closely after switching. Synthetics have different seal swell/shrinkage characteristics than mineral oil types, which could lead to some amount of leaking (this problem has generally been solved). Also, synthetics have a better inherent detergent ability which cleans out crud in an engine, which in turn could open-up leakage points in an engine.
Bottom line? give synthetic a shot!
- Change intervals for synthetics? You would be wasting your money if you did it at 5k-6k or less miles. Certainly 3k is way too soon.
