Posited a long long time ago on here that an SAE30, properly additised (AW, detergents), S rated, but not generally with FMs, VIIs, and PPDs would make a pretty benign "top up" oil if you wanted a one top off meets all spec. At the time, the two that I had in mind were Delo 500 SAE 30 (used in Hadfield's Merlin engined '55 Chev, and also for an entire change in my E30), or Delvac 1630 (alas, unavailable in less than 205L quantities). I still reckon that an SAE30 isn't a bad idea for a top-off oil, but bought 2.5L of this stuff a few weeks ago. Added to a 20, or ILSAC 30 heading out of grade at the end of an OCI, it will help thicken it up. Can't overly thicken a 40, or an A3/B4 (30 or 40), as it's already at the HTHS minimum for those grades. And it's red...just in case you forgot that you topped up.