Can I use 15w40 synthetic oil in my 2015 vw jetta tdi and how long can go between oil changes?
15w40 oil is way too thick for this car. Just stick to the oil that is mentioned in the owners manual, which will probably be 5w30 or 0w30.
Please read the owner's manual for information on the correct oil to use. 15w40 is much thicker than 5w30 or 0w30, and is not typically used on modern cars. Also take note of the grade i.e A5/B5. Matching grades ensures total protection.
Fully synthetic oil will be recommended by the manual as it is better than semi-synthetic in every way.
Oil should be changed every 12 months at a minimum. Old engine oil lubricates less and increases engine wear at a faster rate.