Pretty much on the money there grumpy. SG was the current automotive OEM spec from 1989-93, SJ was 1998-2000.
To elaborate a little on what Bushie has said:
For small engine service, a later API spec is not necessarily an advantage. The SH and later specs have restrictions on the allowable Phosphorus level, as it isn't good for vehicle catalytic converters. It doesn't 'produce a poison', but rather inactivates the metallic catalyst coating, a phenomenon known as 'catalyst poisoning'.
But the most cost-effective anti-wear additive for engine oils is a class of chemical compounds collectively known as 'ZDDP's', which contain both Zinc and Phosphorus. So this would be a good reason for Penrite to stay with the earlier spec, to keep the anti-wear additive levels up.
The problem with multigrades in these engines is not sludge formation, but rather that the 'viscosity index improver' additives, which are long chain polymer molecules, can get sheared into shorter bits, which don't work nearly as well. Much more so than in multicylinder engines. So in service, the oil will tend to gradually revert to the lower end of its viscosity range [the one with the 'W' suffix], and more will get burnt.
Thus a straight SAE30 is still the go, for these engines.
FWIW, you can set up e-mail alerts through the Supercheap website, to let you know when specific products go on special.
