Usually, the slippage in CL is $10 per side, or $20 total. I use $30 in my work to account for some extreme slippage situations I have encountered, and also because Crude rolls every month, and many times my strategies are in trades overnight (not intraday), and therefore need to roll. So, the $30 slippage number helps absorb some of those roll costs, which otherwise are not accounted for in a standard continuous contract backtest.
Overall, the $30 may be a bit conservative, but I like to err on the high side and be pleasantly surprised if it is less than that...