If the local garage does it then it might affect the warranty. But if you can prove that the correct oil (usually 5w/30) and the correct Ford oil filter and sump plug were used and that the old oil was drained, not siphoned, all detailed on the invoice, then I don't think you should run into problems. It's high time someone set up a nationwide chain of lube shops doing nothing but oil changes for all makes and models using the correct lubricants and filters. That would put an end to some German manufacturers charging more than £300 for a simple oil and filter change.