ST Line/Zetec S are 10mm lower than standards. ST is 15mm lower than standard but with much higher spring rates. Eibach are one of the few manufacturers that have different lowering springs for ST and other Fiesta models, so make sure you choose the correct model.
My Ford dealer in Thailand routinely uses the Ford spec 5W-30 as it is 1/2 the price. According to them the warranty is maintained as it is the alternate oil specified in the manuals (user and service). Of note is that it is always hot here and there are no stop/start equipped 1.0 Fiestas.
A Ford dealer, if they will do it at all, will charge a fortune. Download Forscan, get a modified OBD2 dongle, and do it yourself. Heaps of guides on here.