1998 Mercedes Benz S500 valve/lifter tap

Hi. I recently purchased this car. I have the history and it has run fine. I only put about 1200 miles on it and decided to have it serviced. My mechanic was not sure if it had sinthetic oil previously or not. I have driven it about 10 miles since the oil change and it has developed a velve or lifter tap that it never had before. Could it be due to the wrong oil put in it. I just descovered this noise and it is the weekend so I can not get in touch with my mechanic to ask him what type of oil he used. I went through the previous records and can see that Mobil 1 has always been used previouslly. I do not even know if that is a sinthetic oil or not.

What could be the cause of this problem and what can I do to fix it.
Thanks,
Bob