What the guy said is somewhat right. Since you have been using mineral oils before and 10w-40 is semi-synthetic which contains detergents and will flush all the carbon/soot deposits off and any deposits around piston rings will loosened and disappear which effectively reduce your compression.
That is why, it is not advisable to (use) semi or fully synthetic oils after a prolong use of mineral oils which in your case is 14 years.
So if you haven't done any flushing in the past 14 years or so then it is really not recommended, if you do then you might kick the muck up.
Cheers,
Kaleem