For Shell - If you don't mind adding the CarSales App to your phone, you can get 8c off petrol on top of your Coles Express offer. There is also the offer from MyCar car servicing (not sure I would recommend them though) who offer you a "Shell Secret Saver" app for two months during which you can use up to 20 times their 10c off Shell petrol. However, you can't stack this offer with the CarSales Shell offer because both offers are via a company called Viva Energy and can't be stacked.
For Ampol - Ampol has a 6c off offer for those who own a Toyota and download the MyToyota App every 24 hours with the barcode updating every day. You can also buy digital gift cards via Shopback for 3% off which at today's petrol prices, is another 5c off. Some Ampol petrol stations also allow you to get 4c off if you have a Woolworths petrol offer on your Woolworths rewards card. However, some Ampol stores only show an Woolworths Rewards card banner but not 4c off. This means that you can earn Woolworths Rewards points, but you can't redeem your 4c off.
For Caltex Woolworths - You can buy WISH giftcards from places like RAC or the Entertainment Book for 3% off which can be used at Caltex Woolworths petrol stations. Note that you can't use Woolworths gift cards at Caltex Woolworths petrol stations, only WISH giftcards or WISH egiftcards. If a Caltex Woolworths petrol station becomes an Ampol, I don't think you can use a WISH card there either. Confused? So are we. If you notice any other offers or exceptions floating around, please leave a comment below