I believe it will begin gaining weight and ultimately develop heart problems

God Bless U!!

Maybe due to rising cost, the owner is not giving it enough petrol and it has therefore been forced to survive on engine oil.

In a car engine, oil is used to lubricate most moving parts.For your case, it seems like oil that lubricates the piston movement in the cylinder is getting into the combustion chamber and getting burnt together with the fuel. You need to do a compression test to determine whether there is compression leakage in the cylinder. This can be done in a good garage. If there is a compression leak, oil can also get past the rings if the rings or cylinders are badly worn or damaged, if the cylinders were not honed properly when the engine was built (or rebuilt), or if the rings were installed improperly.
When a newly-built engine is first started, the rings require a certain amount of time to "seat" or break-in. If the rings fail to seat properly, the engine will use oil. This may be the case if somebody applied the wrong finish to the cylinders, failed to clean and lubricate the cylinders properly before the engine was fired up, or didn't use the proper break-in procedure.

If the rings and/or cylinders are at fault, the engine will have lower than normal compression readings.

In some instances, worn rod bearings, excessive bearing clearances and/or excessive oil pressure can splash too much oil on the cylinders causing oil to get past the rings.

The cure for worn rings and cylinders is to overhaul the engine block. The cylinders have to be refinished and new rings installed to regain good oil control.
Also, make sure you are using the appropriate oil recommended by your manufacturer, check for engine oil leak under the car, and good luck!!

Which car, yourself or a motor vehicle? If youself, try alternative exercises that do not involve discharge of oil.

sorry if I repeat some responses. Main causes of using up too much engine oil:
1 - Oil seals leaking - take the car over a pit and look at the underside - from the top it might not always be visible.
2 - Worn oil rings on the pistons - but if you are not leaving plumes of smoke behind as you drive, then its not the oil or compression rings
3 - Breached cylinder head gasket - first the cause of this is engine over heating or extremely high revving (e..g did you get stuck in mud and took the RPMs to insane levels?). A tell tale sign is to look at the coolant - even the radiator lid - if you see some creamy substance, thats a mix of coolant and oil.
4 - Worn valve seals - a tell tale sign would appear when starting the engine after its been idle for a while - even 10 mins - if on starting you see some whitish smoke from the exhaust, which stops after a minute or so, then your valve seals are gone. Reason is, when the engine is at a stand still, any oil at the tappers/rockers, seeps down the valves into the cylinders.
5 - Check your oil warning light and make sure its properly conencted and not loose. Now this is IMPORTANT. Its possible that you may have had a loose connection to start with and figured you were low on oil, when in fact you were not. I know this has happened to me. The problem is, if you add oil, especially going by the light alone, and not waiting for the engine to cool and check the level (if you check the level when the engine has just been switched off, it will show a lower reading since the oil is still making its way back into the sump). So, if you check shortly after the engine is turned off, and then add, the problem you are creating is too much oil. Too much oil? can't be that bad? It is. More oil will cause higher engine oil pressure which in turn will breach the oil seals (crank shaft oil seal, cam shaft oil seal, etc)

HTH.