I have run DIESEL GRADE 10-40 or 15-40 ( XPower Stuff ) Engine Oils in the Petrol Engines of used cars I have bought. Been doing this for at least a dozen years. In every instance, it has been beneficial to the engine to some extent, sometimes spectacularly so.
Good quality DIESEL grade engine oil contains Detergents which are slow acting. Diesels run dirtier than Petrol engines. Those slow acting detergents can cleanse those sticky Hydraulic Tappets ( Cam Followers ) and many other engine parts over a few thousand miles. Reducing the Cam Followers to a near silent rustle. Then drain the Diesel Oil HOT and fit new Oil Filter. The return to the factory recommended 10-40 Semi-synthetic.
EDIT TO ADD @ 01:45
If at some stage the cylinder head has been skimmed, that involves Camshaft removal. On assembly it is necessary to apply sealant to the narrow oil channels which supply oil under pressure to the camshaft bearings and those Hydraulic Cam Followers. I say carefully because it's far from unknown that an excess of sealant can be squeezed into and block those small oil supply channels and starve the moving pasts. Such as the Cam Followers ( OK .. tappets ) which can become partially seized in their bores and that will result in it or them sounding like a neglected high mileage old diesel. One I know had noises so loud, was so bad, we thought it was a Big/Little End making all that row. We were wrong, it was caused by oil channel blockage.
I am NOT recommending this but here's what I did two years ago to my daily driver MG ZT 1.8t Cylinder Head during overhaul. As the whole engine was filled with a thick oil-coolant mix from the damaged cylinder head gasket in previous ownership, I wanted all that stuff out of the engine. I bought 5 Litres of Diesel Fuel, put it in the Petrol Engine's sump and ran the car gently for six miles only. Then drained the Diesel Fuel ( It's actually an Oil so has some lubricating properties ) and what came out was all that remaining filth left in the engine after previous draining. Two years and several thousand miles later, my ZT's engine still works beautifully. The oil on the dipstick is still reasonably clean and clear. without using that Diesel Fuel as a flush, the new oil on the dipstick would soon become discoloured from the muck left in the engine.
This was the oil-coolant-mix muck which came out of the engine when I drained it for the first time. Not nice :~
This picture shows the colour of the Diesel Fuel used to flush the engine and what it looked like when drained hot after a six mile gentle run :~
This picture shows what else came out of the engine when draining the bowl ~ small solids dislodged by the Diesel Fuel. :~
This for me is the clincher. Oil still relatively clean and contaminant free after several hundred miles following the Diesel Fuel flushing treatment :~
I repeat, I am not recommending this treatment. I took a calculated chance that things may not work out. However, my sensible gentle six mile run ensured it would be unlikely to harm the engine's internals.