SWMBO's JK is now our daughters, it has 320,000 on it and is still running well. It does use maybe a litre of oil between oil changes but that's less than a new v8 Toyota
I'd not be fretting too much about the engine, just drive it and enjoy it. BTW tell your grandson that our 17yo daughter had a manual soft top TJ as her first car
Regards,
Tote