If it is the synthetic blend, you should change the oil every 3,000 miles. If it is a full synthetic, you should change the oil every 5,000 miles.
Every 3,000 miles Ford recommends every 5,000 miles with normal oil or synthetic blend. You can go up to 7,000 if you use full synthetic.
If you drive normally and use full synthetic you should change your oil every 8,000 miles. You should change the filter at the same time and change your air filter every 30,000 miles.
The best oil is 10w-40 full syn. If you go by the oil change indicator lights, use full synthetic. If you change it at half the interval, regular oil is okay, but I still recommend full synthetic. It should have a rating of ACEA-A3 rated.
For the 2003 1.8T Audi TT you should consider oil changes every 5,000 miles with full synthetic oil as turbo charged engines run hotter when enjoyed. Also, always change the oil filter when you change the oil.
You can mix part synthetic oil with full synthetic oil.
OFTEN!!! never go over 3000 miles and only use full synthetic 5w40. I personally change my oil every 1500 with synthetic, some say that's crazy but I have close to 100k and have never seen the oil light come on. I have known many who have used regular oil or not changed it enough and the engine and turbo failed at 40000, costing $9000
I usually use 10W-40 full synthetic for summer season and 0W-40 full synthetic for winter season.
I work for a lube distributor. It is fine to go from synthetic to synthetic blend. Here's the difference: No manufacturer will actually release how much synthetic is in their synthetic blend formula. It may actually be closer to a conventional oil than a synthetic oil. The benefit of synthetic is that it offers better protection for your vehicle/ machinery. The secret is that it only makes a true difference if you're one of those people who doesn't change your oil regularly. If you choose to go synthetic blend, make sure you keep up with how often you change your oil!
Who told you it was not good? You can and should use full synthetic in a 150cc air cooled engine.
5w30 full synthetic
FWIW, my Honda dealer here in Mpls told me the Honda brand 0W20 oil is SEMI-synthetic and NOT full synthetic. That seems logical to me because they charge $39 for a 0W20 oil change and to get full synthetic almost anywhere it cost you around $60 for an oil change. Full synthetic Mobil 1 0W20 cost $6.50 a quart at Walmart, so you are already over $33 just for the oil. Add labor and a good filter and your at the $60 mark.