It's not appropriate to buy significant amount of stock for Chevron or any other company you work for. Enron employees learned that back in the day. Beneficial to buy stocks of companies you understand and believe in their future potential. While CVX has definitely a good path forward compared with the average oil company, since your income/job is tied to CVX don't overexpose your financial future by loading on stock.
A small percentage is appropriate, everyone has his own comfort level, personally more than 10-15% is too much for me.