Diesel oils are useable in petrol engines, but if signs of low temperature sludge formation are seen, then switch back.
Petrol engine oils are not suitable for diesel engines in the long term because of the lack of detergency/dispersancy predominant in a diesel, and also a lack of high temp sludge format.
modern petrol oils do have aggressive detergants. stick some in an old engine which has been run on mineral oil and over time watch the oil leaks starting to appear.
10w40 oil is a std lubricant for all normal engines, the modern cars run 5w (thinner) oil due to the finer tolerances on the oil galleries. So the 15w is a thicker oil which should be ok for the jeep but there might be a risk of top end wear if used onver time due to the delay in lubrication as the oil pump tries the ush the thicker oil around the system. once warm it shouldn't be an issue.
dont panic and decline the use, watch the oil pressure gauge when you start the engine and see where it spikes and settles to then swap to the 15w and repeat. depending on the sclaing of your gauge there might be a slight increase in the reading.
On old engines tbh it's not a big concern as the choice of grade would have been the 10w and 15w and the temperature derrivatives
