Cat specific oil is available from qualified service centres. It is a synthetic with many additives and carries their warranty for performance. Not having their oil in the crankcase will void their warranty. The oil costs ~$6 per quart/litre.
Scared yet?
You can use NON-detergent straight 30 weight oil with no problems. Always check the sight glass to ensure there is no water in the oil to make sure you do not see a butterscotch colour. If it is a direct-drive application you can use 80-90 gear lube for greater resistance to heat transfer.
In GENERAL, synthetic oils provide no benefit in pressure washer/small engine applications. Synthetic oils are more resistant to viscosity breakdown and heat and therefore allow for extended drain intervals. Unless you are going to sample your oil to ensure you get maximum benefit, I strongly recommend you use a high quality regular oil.
Do not get me wrong, I love synthetic oil for my diesel Jetta and with testing am changing the oil every 25,000 miles (40,000 kms) with only filter changes every 6,000 miles (10,000 kms). NB: synthetics are not oil but parrafin.
Hope this helps? Cheers.