I think the ex owner was a little remis on checking oil levels and such. I like the explanation below for burnt valves.
It simply means a half burnt valve thus preventing it from sealing against its seat
When there is no sufficient cooling in the engine it may cause the seal of the valves to give way(exhaust valve generally), gradually over the course of time the exhaust gases escape even without the valve being fully open thus increasing the risk of further damage. A partially burnt valve will be difficult to notice but will produce anomalies in the fuel economy.
•Exhaust valves are generally hotter than intake so they are more susceptible to burning , as stated above , if your valve is burnt then the exhaust gases will escape even though valve is not open, you can do a compression test to adjudicate if the valves good.
•Cylinder Misfire, if your head gasket is good and still cylinders misfire then there is a high chance that your valves might be burned since it causes compression loss.
•Slight power loss, depends on the vehicle whether it will be noticeavle or not.
•Smoke in the exhaust.
•Engine overheating due to insufficient cooling.
•Engine running lean(one of the common causes).
•Having LPG kit fitted which on prolonged use causes less lubrication to the valves.