Devon Aygo has recently supplied the following information on oil requirements:
"The issues over which oils are used is caused by a European law called "Block exemption" the same law that states that manufacturers can't insist on main dealer servicing, also stops Toyota from insisting what oils are used by anyone wishing to service a car including their own dealer network. All Toyota are allowed to do is list a set of specifications that the oil used must meet they can also list a "preferred" oil. For example I have listed below the requirements Toyota have for the mkiii Prius, a car that has caused a lot of posts over the oils that have been used during servicing. This info is taken from Toyota's Service data sheet for European Prius mkiii only!
Other listed options:
These grades above are listed by Toyota as they meet the following required specifications:
API grade SL or SM multigrade engine oil
API grade SL “Energy-Conserving”. SM “Energy-Conserving” or ILSAC* multigrade engine oil.
Toyota can only require the oils used meet the minimum specifications above. So long a the oil used meets the above the oil grade is not enforceable.
At present Toyota have "asked" but cannot enforce the dealer network to use the following,
Mk3 Prius, Prius plug-in, Prius +, Mk1 Auris HSD, Mk2 Auris HSD & Yaris HSD 0w20
Mk1 & Mk2 Prius 5w30
Optimal drive petrol models 0w20
Other petrol engines 5w30
Diesel engines incl Optimal drive 5w30
Diesel engines with DPF/DPNR including optimal drive 0w30 or 5w30 meeting C2 low ash
1.6 1WW & 2.0 2WW ( BMW ) 5w30 meeting C3 low Ash
1.6 DV6 & 2.0 DW10 ( PSA ) 5w30 meeting C2 low ash