Top tip for market watching (on Amazon at least) have a look at a website called camelcamelcamel that indexes and tracks prices of items. You can get it to email you once an item drops below a certain price threshold. On Google Chrome I have an extension called 'camelizer' that makes it easy to check amazon listings.
Link below shows price history:
https://uk.camelcamelcamel.com/product/B0043R0L0A
P.S. My personal best for oil was £26.80ish for a 4litre castrol edge titanium 0W-30. Some lucky buggers had it for £22.90!
