Royal Purple Engine Oil – $25 back, mail-in promo

Need an oil change soon and I’ve been using Royal Purple for the last few years since I got my vehicle. Between Walmart and Canadian Tire, depending when, I usually find the best deals for this brand in the $50 range. Went online at Crappy Tire and it’s 54.99 – down from 71.99 – and there’s a link to Royal Purple’s promo page with this Canada-only offer. Limit 2 per person/household, etc., so potentially $50 back/per. I think this is an excellent deal for one of the best performing synthetic engine oils out there. Yeah I’m sure there’s better, but for this offer I think it’s hard to match.

Question: How long can/should you store engine oils?