trying to understand your question,
1)300 hp is that measured at the rear wheels by a dyno or is that just published numbers?
a stock gt depending on the year runs about 260 to 280 rwhp, so if you are stock stick with the octane recommended by ford, higher octane will do little for you.
2)if you are modified, (CAI, exhaust underdrive pullleys, tuner chip) you may be running 310-320 rwhp (my last gt was in this range) and you should definitely be using 92 octane, 94 would be better, not so much for more power but because tuner chip had set an advance on the engine for higher octane..
3)at only 300hp you are not supercharged, so running higher octanne is non-issue.
now as for the oil that has nothing to do with performance but best lubricant is a top quality synthethic like motul or quaker state 10-30.
bottom line unless your engine is tuned . raced ready to accept really punchy octane numbers stay with recommended fuel levels. if you enhance the engine and move into higher performance territory you may will require extra octane (by way of 94 octane gas or octane boost in a can or meth kits or race gas).