I've had good luck thinning with mineral spirits 50/50 and wetting some wet/dry 400 grit paper with the mix and sanding to produce a slurry that fills the grain. Wipe cross-grain with a rag while wet. Let it dry, repeat until satisfied with the fill level of the grain (a couple will produce a decent surface, more will fill completely). You might try mixing in some oil based stain with the 50/50 mix if desired.
After the wet sanding/application process, I thinned about 75-80% Trueoil with remainder of mineral spirits and applied with a rag. Sand with 400-500 grit as necessary after several coats, steelwool with 000 or 0000. Burnish with a dry rag or polishing paste.
