If you're going to race the vehicle, you need a oil pressure gauge as a minimum plus an oil temperature gauge if you want to see how much heat stress you're putting on the oil.
Given the RPM band that your engine will see, a high mileage oil with it's additional seal conditioners, dispersants, and detergents isn't going to provide the best protection you'll need.
You didn't say what type of bearing/rod clearances you have, but blindly using a 15W-50 oil is not the answer. If you are using a stock engine with low mileage, you won't need such a thick oil as it would rob you of HP better spent driving the car.
If this is a high-mileage, stock engine, your best bet is using a comparable weight racing oil recommended by the manufacturer.
If your bearing clearances are excessive, you may need a thicker oil, but without an oil pressure gauge who knows? (For example, when I raced a GT-1 Corvette in the 80's, my cold cranking/idle oil pressure was 85+ PSI until the straight SAE 30 racing oil reached 180F then the oil pressure was maxed out at 60 PSI. This was a purpose-built SBC with tight, i.e., minimum, clearances.)
If you don't get an oil pressure gauge or an oil temperature gauge, I recommend using Valvoline VR-1 racing oils if you intend to dump the sump every few races or RP (stock or racing) if you intend to use the oil for the entire season.
Hope this info helps! Good Luck Racing! (Have Fun!)