Sorting out my garage and came across 2 x 4ltr containers of the oil used on my bike both had been used for previous oil changes on the bike (same make and viscosity) 3ltr used in each change leaving a 1ltr in the container the date stamp on each container is May 19 and November 21 ( oil change last year was done at bike workshop) question can these oils be used still/ combined or should I take them to the local tip and buy new oil for this year's change ?
It’ll be fine - buy another 4 litre bottle of the same stuff and use 1 litre of that with the remnants of the previous.
if by used, you mean left over then it is pretty safe to use if it is kept is a sealed airtight container