* November light synthetic $3.90/bbl over WTI
* WCS quoted at $14.75 under WTI
* Tesoro maintenance adds to refinery outages
CALGARY, Alberta, Oct 12 (Reuters) - Canadian cash crude prices extended their slide on Friday as refinery maintenance picked up, reducing demand.
Light synthetic crude for November delivery last sold for $3.90 a barrel over benchmark West Texas Intermediate, compared with $4.25 a barrel over WTI on Thursday, according to Shorcan Energy Brokers.
November Western Canada Select heavy blend was quoted at $14.75 a barrel under WTI, a 10 cent deeper discount on the day.
Prices have weakened gradually since the start of October when the spreads prompted some shippers to forgo planned shipments to the Cushing, Oklahoma, storage hub as they could not compete with the local prices when transport was factored in.
On Friday, Tesoro Corp said it was conducting planned maintenance at its 120,000 barrel-a-day refinery in Anacortes, Washington, a big buyer of Canadian crude.
Among other operators, Royal Dutch Shell Plc said Wednesday it was taking an unspecified unit at its Scotford oil sands upgrader down for maintenance..
The 255,000 barrel-a-day upgrader, located near Edmonton, Alberta, processes bitumen from Shell’s Athabasca Oil Sands Project.
Suncor Energy Inc, meanwhile, is scheduled to start planned maintenance at its 93,000 bpd Commerce City, Colorado, refinery this weekend. One of the crude units is slated to be down.