Liam, that works so long as you have oils which are in linear steps ... most oils sold as *.wt [numbers like 25wt etc.] aren't but the oils sold as cst (numbers like 250 are the actual measurment), for example Kyosho and Xray I believe, are linear.
The best way to save money in this sport is make some friends and travel together, fuel costs (for the 1:1 car) are now the biggest cost to racing ... that cost is even more than tires for a meeting usually for me
Make sure you cut tires off rims before using your prefered degluing techneque so you can be 100% sure you can save the inserts as well as the wheels.
Last one is buy a car that doesn't break or wear out much and doesn't need upgrades from day one ... sometimes a few extra ££s spent on the kit can make a real difference to you overall costs. For example the majority of my Kyosho equipment is about 3 years old now and still going strong!