In the first few levels, the round only lasts a few months; therefore, you have to hurry to get as much oil sold as possible. It is very likely that you won't be able to get all oil above ground in these first few levels. because of the time and technology constraint, so...

• send out 2-3 drowsers immediately
  
• place multiple rigs above each oil pocket
  
• sell constantly, without worrying about price too much

This stage has no special elements, just oil. Set yourself a threshold price under which you won't sell. I usually set this to a value between 0.85-1.00$. Though sometimes, prices don't go very high and you have to settle for what you get. Start by placing 2 drowsers and placing the oil rigs where they stop; then, build pipes straight down in 150-200$ lengths. After that, when you have the upgrades, using a combination of dowsers and moles to find oil is best. First send the dowser then the mole to that location. This avoids blindly looking for oil.

Here, there is oil and rocks. Rock is an obstacle you can't always avoid. Get the rock drilling upgrade in the factory right away. Only upgrade a drill once you hit upon rock. Connect that drill to other drills with pipes if it's cheaper than buying separate upgrades, which it usually is.

In the third stage, the strategy changes. Here, there is oil, rock and gas. Gas allows to raise prices by channelling it to one of the companies. Moles become much more important as they can sniff out gas. You don't want to tap into a gas pocket accidentally as this will let the gas escape. Close the valve immediately if this happens. So, get the company link, valve and mole upgrades ASAP.

To start, send out 2 dowsers and 3 moles evenly spread. Aim to have as much oil as possible in storage towards the end of the level. From here on, the placement of rigs and storage is important as you want to maximise the amount of oil you can store. Once all oil has been depleted, close the valves at the rigs, connect to the company link that is closest or has the higher price at the moment, and connect to the gas pockets. Sell oil at the resulting high prices. It is best to have at least a month to do this.

The fourth stage is the same as the third; however, you can now expand gas. So that it has the most time to grow, find the gas as soon as possible using moles or scanners, whichever you prefer, and set it up for expansion.

Before selling, price is 0.97$

After price boost, the price is 2.50+$

This fourth stage is where the achievements The Big Haul, Good Deal and Great Deal are achievable.

• After sending out your drowser/scanner/mole at the beginning of a level, place a horse cart immediately. You don't want to end up with oil flowing, no cash and no cart to sell the oil, forcing you to take a loan.
  
  
• The horse cart upgrades correlate with the pipe width upgrades. After upgrading to a 2nd level pipe, upgrade your horses too so they can keep up. The same goes for 3rd level pipes.
  
  
• There is a 1000$ bonus for early withdrawal so if you've finished a level early, click on the date and withdraw land lease.
  
  
• If your dowsers are fully upgraded and they walk off the map without finding oil; then, there are no more untapped oil pockets. If the dowsers aren't fully upgraded, the oil pockets might be there but deeper than the dowsers can detect.
  
  
• There are four possible bonuses each level, each worth 1000$: all oil depleted, all oil sold, all gas depleted and early withdrawal.
  
  
• Don't ever take out loans. The only reason to take one out is if you accidentally spent all your money before buying land or forgetting about the 2000$ startup cost. To avoid such a situation, buy land before anything else.
  
  
• At the beginning of stages 3 & 4, before bidding on land, give a diamond to Anthony in the Saloon so that he points out land with a lot of oil.
  
  
• Dowsers detect oil below them, but the oil pocket may be 1 cm to the left or right, not necessarily straight down.

I hope this guide was helpful. If you'd like me to go more in-depth or have any questions, please write in the comments below.