I currently have #10 (about 5/8") running to my remoter filter. The fitting size for the Mercruiser oil cooler I'm installing is #8 (about 1/2"). Is this smaller line size OK?

If you are using AN line, then there are #10 to 1/2" adapters available. That's what I'd use.

I've always used -12 for engine oil, -10 for tranny fluid, and -08 for fuel. I wouldn't go below a -10 on motor oil. I don't know if a -08 would actually limit flow volume, I'm sure some Engineering types can recall the formula to figure it out though.

Weird thing is I'm using a oil cooler off of a 454 mag and it has #8 inverted flare fittings built into the cooler so I'm thinking that's OK. I'm using this on a 400HP small block that I haven't had a cooler on in the past but I don't want to choke off the oil supply.

Use #10 lines with 1/2npt fittings on your oil cooler and engine ports. The fittings are common and have a much larger i.d. than fittings for #8 lines.

I can't use 1/2" fittings on the oil cooler. It is threaded for #8 fittings. I guess this is OK since it is off of a big block mercruiser but I want to make sure. Is anyone running #8 lines to there oil cooler?

Strange, my 454 mag coolers had 1/2 npt threads. The system was plumbed completely with #8 lines and fittings but I don't think that's enough for a big inch engine or a supercharged application. You lose a lot of pressure going through 14 feet of hose, fittings, oil cooler, remote oil filter, oil thermostat.

I use a -10 with 1/npt fittings on my cooler and remote filter.

I used 12 you may get away with 8

No stay with the 10s don't step down. It is possible for the filter bypass to open up when the engine is cold if the restriction is too great.

Roby