Depending on what year your dads Northstar is, he should use either a 10W-30 or a 5W-30, 5-30 is for 2000 M/Y or newer. Next, the Northstar engine was designed and validated for 300,000 miles with CONVENTIONAL oil. So, he can use whatever he wants. If you ingore the old wives tales about extended break in and increased seal seepage with synthetic, you will find there is not much difference. None that you can tell from behind the steering wheel anyway. More importantly he should use an SL oil or an oil with very little VII to keep the rings from sticking. Also a GF3 or better to keep his gas mileage up around 27 - 28 mpg. I got my best fuel economy with Mobil Drive Clean conventional oil (not blend) and my best oil consumption with Pennzoil 10W-30 in my 1999 STS. I currently am running the much touted Castrol German 0W-30 for winter. It is a heavy viscosity synthetic that probably will meet or exceed all 10W-30 specs both when new and after 6000 miles are on the oil. Now for the REALLY important stuff. #1 ONLY PUT 7 QUARTS OF OIL IN THAT ENGINE! That should take the oil level up to about the "ADD" mark...don't worry, it will be just fine with 5 quarts, the extra oil was put in there to meet the 1G lateral acceleration specs for the engine. Unless your dad is running at Indy, he won't need the extra. The book calls for 7.5, but some people keep the oil level at the Full mark when cold, that takes about 8.5...the extra oil is quickly siphoned off by the PVC valve and burned. This results in stuck rings and excessive APPARENT oil consumption. #2 FLOOR THAT SUCKER ONCE IN A WHILE! This engine was built to be run hard, if he drives it like a, well, DAD he is going to stick the rings for sure. The maintenance maneuver for unsticking the rings is to a) floor that sucker on entrance ramps to interstates. b) Start with tranny locked in second gear, take it up to redline (about 75 mph) then let off the gas, allowing the motor to do the braking. This will get any carbon out of the heads and free up most stuck rings. By the way, to get 300,000 mile out of an engine, they had to be designed to use a little oil all of the time. That is why on this and other new design engines, 1000 miles per quart of oil is considered acceptable. I know, a lot of stuff to get you mind around, and it goes against most of the advice you will get about 3000 mile oil changes, thick oil, high mileage oil, the whole nine yards. Try to change your way of thinking about all this, follow the advice here and the dashboard oil change indicator for intervals and you will be just fine.
