Assuming your mechanic is a "normal" mechanic....
when the oil change is done, they remove 2 drain bolts, both of which have copper washers between the bolt head and oil pan. Oil drains out. Mechanic puts the bolts back in, then removes the oil filter and replaces with a new one. Then he removes the fill plug and pours 8 quarts of oil in, roughly. Reinstall fill plug, start engine, then check the filter for leaks. Shut it off, wait about 5 minutes or so, double check the oil level, add if necessary by removing the fill plug again and adding a small amount until the oil level gage (dip stick) reads full. That is how an oil change is done.
Not a single seal or gasket is touched. The oil filter has a square cut o-ring on it. Occasionally the gasket will stick to the filter adapter (not to the block since the filter doesn't go on the block itself, it goes onto an adapter that is bolted to the gearcase cover on the front of the engine). If the gasket stays on the adapter, the tech should always check, I look at the old filter after I pull it off. If the filter has the o-ring on it, it's good to go in the trash. If it's not on it, first thing to do is look at the filter adapter, it's usually there, or in the drain bucket when it fell off. Either way, it needs to be found (or like they say in Georgia "fount"). If it is left on the adapter, the new filter's gasket will seal against the old gasket, but there is such a large area for oil pressure to apply force to that it usually blows out one or both gaskets, normally (about 90% of the time), it happens within seconds of starting the engine. That would be a big BIG leak, by the way...8 quarts of oil in maybe 20 to 30 seconds.
So assuming he did it correctly...filter gaskets checked and not "double gasketed", then there is another leak, which would be unrelated to the oil change (unless there was something else done during the oil change). If that is the case, on an L48, the first place I look is around the sides of the engine. Also on the front of the block. There are welch plugs that plug oil galley holes in the block. Those holes were drilled as part of machining the block. It is not uncommon for those plugs, one or more, to come out. A few of them are behind the front axle frame and cannot be seen without removal of the front axle frame assembly, which also means at least partial removal of the loader and backhoe, then the subframes, then the front axle frame. Have seen one or two of those plugs come out while engine running, operator did not know, and destroyed the engine. It is an expensive repair. If that was the case with the OP's tractor, it would be completely unrelated to the oil change unless the oil change entailed more than a simple oil change, at which point we aren't getting the entire story. If that IS the entire story, oil change only, then any leak beyond a double-gasketed oil filter would be unrelated to any other leak, and IMO would be considered an unfortunate coincidence that it happened not long after the oil change was done. Welcome to the life of a technician. "Hey I just had my truck in for oil change and now the tire's flat....you pieces of garbage grease monkeys are going to PAY for that tire that you screwed up".....believe me...it happens quite often.
Had to walk a mile to the nearest pay phone to call and raise holy hell. About three quarters of the way to the phone my boss happened to come by and gave me a lift back to the speed lube place.
