Jon, to echo and hopefully clarify Blackhand's good warning, the issue isn't whether this oil or that is the right spec, but I've had first-hand experience of changing from one oil to another (both the right/same specification) during a period of service, and it's a very real possibility that the second oil may dislodge carbon which was deposited with the first oil. This subsequently blocks small oilways and bang goes the donk
If you're paying for your own repairs, I'd strongly recommend staying with the oil the engine is running on, and if you're determined to change brands OR specs (even within what's approved for the engine), ONLY put the different oil in immediately after strip/rebuild has guaranteed all the old residues have been cleaned out - not usually possible to be certain of this until full overhaul strip.
You might be lucky and get away with it, but I wouldn't risk my own hard-earned....