I have a couple older Murray mowers with Briggs engines that call for straight 30W oil. I had been using Castrol 30W from Walmart, except now they don't seem to carry it or any other straight 30 weight oil except for the following. They have Supertech 30w rated SN (not the compressor oil) and they have actual Briggs and Stratton 30W oil in the garden section. Trying to keep things easy and not running all over town to get the Castrol, what's the opinion on the Supertech vs. the Briggs and Stratton straight 30 weight?