Definitely go with Metac. Metac is fully equiped to cover most situations, including oil tank storage, breakdowns, etc. From what I hear, Chevron do courses in hotel rooms. Metac course is 2 days per week for 10 weeks. How you can get this amount of info in 4 days and pass is anybody's guess. You will have 10 exams and pass rate is 100%, however, it is open book.
None of the oil boiler courses are geared towards the wet side of the system. It is an oil boiler and oil storage course, certified by Blueflame and based on OFTEC and Building Regulations. Once passed, you will be entitled to register with OFTEC.
Course, I would say is 60% theory (regulations) and 40% practical. A lot of technical books studying is required for the period of the course.