Spray four (8 ounce) microwave safe custard cups with butter-flavored cooking spray. In a large bowl, combine apple pie filling, 1/4 cup splenda, and lemon juice. Evenly divide mixture between prepared custard cups. In a medium bowl, combine oats, remaining 1/2 cup Splenda, and margarine. Mix with a pastry blender or fork until mixture is crmbly. Evenly sprinkle about 1/4 cup crumb mixture over top of each custard cup. Arrange custard cups in a circle in microwave. Microwave on HIGH (100% power) for 5 minutes. Place custard cups on counter and let set for 3 minutes before serving. Serve warm or cold.